So you know the seven deadly sins: pride, avarice (greed), lust, gluttony, wrath, envy, and sloth, right? Superbia, avaritia, luxuria, gula, ira, invidia, acedia.

Except--Latin acedia, Greek ἀκηδία isn't "sloth"--it's not laziness and it's not a cute funny animal. But before we get into story of our most capitalist of deadly sins started off as the least, we need to talk about the whole "seven deadly sins" idea in the first place.

If you've never noticed it, the seven deadly sins are a weird conglomeration of emotions, attitudes, and personal traits. The point of the set, as we have it today, is that none of these elements are actions in and of themselves. They're underlying motivations for all sort of sinful actions.

This was very very useful in Latin Christianity after 1215, when the Church committed itself to getting all Christians to receive the sacrament of the Eucharist once a year. You see, in order for the Eucharist to "work" towards salvation, you had to be sure your soul was clean of sins before you did. So all Christians of both sexes (omnis utriusque...it's a thing) had to confess their sins and receive absolution first.

That meant you suddenly had a whole galaxy, a UNIVERSE of sins that people needed to confess--and that priests had to understand, and to understand how best to counsel them not to sin in the future and to determine what penance to assign. Okay, if you're an overworked priest responsible for two rural parishes a day's walk apart and neither benefice pays you enough to support your non-wife and children so you also work as a blacksmith on the side, you don't have a lot of time to spend on spiritual direction of all your penitents.

So boom--seven deadly sins, a great way for people to organize their sins and the priest to understand them. (By the fifteenth century, there are also the Ten Commandments and the nine alien sins and the six sins against the Holy Spirit and the sins of the five senses and the four sins that cry to heaven and...)

So the "seven deadly sins" were very attractive to Church preachers starting in the late twelfth century. And it wasn't an accident that they used these sins and not some other set that would serve the same purpose (see also: the Ten Commandments, the nine alien sins, the...). The later twelfth century was the crest of a massive wave of monastic revival/reform that revitalized a lot of late antique and early medieval ideals of asceticism, spiritual life, and pastoral care. In the case of the 7DS, the 12C writers got them from all-important pope Gregory the Great.

But Gregory didn't make them up, either. You might say, in fact, he translated them--from a list of seven, previously eight, evils of late antique monks. And not just the individual sins, but the whole concept of the list. "Evil thoughts" or "vicious thoughts" is probably a better way to put it than our concept of "sin" as an action against God.

And this is the origin of acedia--not so much a sins as an orientation. The Wiki article on sloth picks out the exact right quotation from monk-theologian John Cassian to describe acedia:

[The monk] looks about anxiously this way and that, and sighs that none of the brothers come to see him, and often goes in and out of his cell, and frequently gazes up at the sun, as if it was too slow in setting, and so a kind of unreasonable confusion of mind takes possession of him like some foul darkness.

This sounds remarkably like what we would call "depression" today. But it is and it isn't. Of course, there's the history of medicine issue what we delineate as a "mental illness" is a web of symptoms, many of which apply in many cases to most people we say suffer from a particular set of problems; "depression" is a modern thing.

But this still misses what marks out acedia as unique: it's not just despair or listlessness, it's spiritual despair. It's a listlessness of the soul in relation to God, not just the mind. Can I describe it in a word? No, not really--because we don't have this feeling anymore. We in the West turned religion into sets of propositions to accept and reject (religio originally meant a way of life according to Rules); we turned "evil orientations of being" into "sins."

And we turned acedia, relentless and consuming despair of the soul, into not wanting to work so hard.

Just to point out,

You made an assumption about what we "know" by using an example that is questionable, from memory there's a few threads that addressed the Greek/blue thing https://www.reddit.com/r/AskHistorians/comments/2s0rfk/is_there_a_consensus_on_why_blue_does_not_appear/?utm_source=reddit-android

Further reading on that matter may surprise you and there's further links in that which I supplied you.

Regrettably, I don't have an answer to your other point as I've not seen the question before.

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Good timing -- I'm writing a thesis for a masters in Classics on a very related topic. I'll try to give you some cliff notes.

First off, I'll point you toward the work of Classicist David Konstan. In 2006, Konstan opened his book-length treatment of ancient emotions thus:

The premise of this book is that the emotions of the ancient Greeks were in some significant respects different from our own, and that recognizing these differences is important to our understanding of Greek literature and culture generally.

The Emotions of the Ancient Greeks.

here's a review of the book with a good summary.

He has since been following up this work with a number of articles. The very best starting place, I think, would be here: "Affect and Emotion in Greek Literature," 2015. Here's the second paragraph of the intro there, and I've bolded some choice quotes for you

At first blush, it would seem that emotions are transhistorical, and that what Greeks thought and felt cannot have differed by much, or at all, from the emotions as they are experienced and understood today. In a recent book, Kostas Kalimtzis rejects the view that “the anger studied by the Greeks of the classical period was something wholly or partially different from the anger that we experience as an emotion.”3 On the contrary, Kalimtzis maintains that the anger that Homer described, and that Plato and Aristotle analyzed, “was anger, not of the Athenian or Spartan, but anger qua human” (p. 3). Kalimtzis mentions me as one of the scholars who hold, on the contrary, that the ancient Greek concept of anger was “significantly different from the modern.”4 Kalimtzis is right that I side with those who view the emotions as socially constructed, that is, as conditioned in significant ways by culture, as opposed to those who hold that at least certain basic emotions are innate and hence uniform among all human societies. This is not to say that I see no continuities at all; subtending the emotions are certain instinctive responses that I prefer to call affects, and that are common to all human beings and indeed to certain nonhuman animals as well. But I find it useful to classify these sentiments as proto-emotions rather than as emotions in the full sense of the word. In fact, the Stoics held such a view in antiquity, I believe, distinguishing between pathos (emotion) and propatheia (pre- or proto-emotion), and their theory may well have its roots in Aristotle himself.5 When it comes, however, to the emotions proper, including anger, pity, fear, love and hatred, shame, envy, and other sentiments that Aristotle labeled pathê, there is good reason to suppose that the ancient concepts did not coincide strictly with the modern.

So you can see what's actually happening here: Konstan and the Classicists with whom he is in conversation -- as well as the philosophers, psychologists, linguists, anthropologists, neuroscientists, et alii -- are debating the simple, fundamental question: what is an emotion? To what extent are emotions innate, and to what extent socially constructed? You know, the question you're asking too. And the "answer," insofar as there is one, is hotly contested. Konstan believes ancient emotions differed significantly from the modern ones, and his arguments have been very influential in classical circles.

For some pretty good books on the debate, you've got J. Plamper's 2007 What is Emotion? History, Measures, and Meanings and D. Gross' 2006 The Secret History of Emotion: From Aristotle's Rhetoric to Modern Brain Science.

The debate has been boiling across disciplines for the last few decades, spawning the field of "Emotional History" (which takes as its starting point that emotions change over time) and entire libraries of new literature. For an overview of Emotional History as a discipline, R. Boddice's 2018 The History of Emotions is a wonderful handbook.

Here are the foundations of the discipline of Emotional History, according to Boddice (2018, the best book-length intro to the field):

1. Emotions change over time: that is to say, emotions are as much the subject of historical enquiry as anything else;
2. Emotions are not merely the effect of historical circumstances, expressed in the aftermath of events, but are active causes of events and richly enhance historiographical theories of causation;
3. Emotions are at the centre of the history of the human being, considered as a biocultural entity that is characterized as a worlded body, in the worlds of other worlded bodies;
4. Emotions are at the centre of the history of morality, for it is becoming increasingly unlikely that any account of human virtue, morals, or ethics can be devoid of an analysis of its historical emotional context. … Emotions cannot be sidelined as another (soft) category of historical analysis, peripheral to the weighty subjects of identity, race, class, gender, globalism, and politics.

The kindling of the debate has been the developing notion of emotions as rational processes that require self-awareness and understanding of a situation (see Martha Nussbaum 2001 Upheavals of Thought: The Intelligence of Emotions).

I'm going to stress the fundamental rationality of emotions again here, because this is the sticking point for the layman. In pop-culture, we've created a nice 'n neat emotion / reason dichotomy that purports that thinking and feeling are separated. Emotional historians, building on recent advances in neuroscience, have shown how this dyad cannot hold: emotions require thought to be felt. (In other words: anger/love/shame/etc require you rationalizing your situation in order to manifest themselves as such.)

So again, this entire field of "Emotional History" exists with the basic presumption that emotions, to some extent, must adapt to new contexts. This idea is founded on recent advances in neuroscientific understanding of emotion, particularly the developing idea of bioculturalism, the idea that humans are shaped simultaneously by biological realities and social contexts.

So, as far as I understand the science (as I've read it summarized in the sources thus far mentioned): there seem to be innate, amygdalar responses in all humans to perceived stimuli. That's step one, an impulse in the amygdala. Step two: these biological impulses then trigger feelings/affects/propatheia (this category includes things like getting red in the face or shuddering; the terminology here is loose because the discipline is young); the feelings themselves are consistent, but not consistently instigated by the same stimuli, and the affects felt can change depending on the words a community has to describe them. So, somewhere between impulse and feeling, the notion of trans-human constants may be cracking a bit. Finally, step three: a human, once rationalizing their situation, feels the entirety of a thing like anger / shame / love. Thus you have a biocultural being: our amygdala, or our biological selves create a sort of frame which society and context fills.

So you can see again how we perceive of some constants that can still change in different communities, here's W.G. Parrott, 2012, "Ur-Emotions: The Common Feature of Animal Emotions and Socially Constructed Emotions:"

There are many differences between the emotions marah (in Indonesian), ikari (in Japanese), song (in Ifaluk), and anger (in English), but in all of them the ur-emotion of antagonism is evident – all four are aimed at an object that is appraised as interfering in some way with one’s concerns, and all four give rise to a motivation to stop that interference in different, culturally specific ways.... The recognition of these components across cultures leads to the intuition that there is something universal about emotions, but it is a mistake to suppose that there exist universal ‘basic emotions’ – marah, ikari, song, and anger are not the same emotion! Rather, it is the presence of the ur-emotion of antagonism that provides the intuition of universality...

Arguing against this understanding are the Affect Theorists. This cabal is the most popular outside of the academy, and you have probably seen their work shared on Reddit before (see: P. Elkman, A. Demasio, building on older work of Silvan Tomkins). Affect theory hypothesizes that there are X number of basic human emotions that correspond with specific, cross-cultural, trans-historical facial expressions. What other scholars call affects or feelings, this group dubs full-fledged "emotions." They reject social conditioning and cultural change over time. (So you know my bias: I've read a lot more critiques of affect theory than I have the theorists themselves, because the entire notion of Emotional History somewhat relies on a critique of AT. I am trying to present a balanced picture of the debate, but want to be honest about my background and preferences.)

Again: these people are all experts in their fields, and they fundamentally disagree on the answers to your question. [con't below]