Throughout etymology, there are continual references to the left side of things or the left hand being unclean or otherwise stigmatized. Sinister, for example, comes from a Latin word used to denote the left. Words in Romanian (stângaci, also used for clumsiness or awkwardness), French (gauche, also meaning clumsy), and English (from Old English lyft, meaning weak) also have derogatory associations with their words for left.
Recently, I saw a post on r/quityourbullshit which indicated that Greeks share/d a similar disdain for being left-handed, as the left is referred to with a word that can also mean “the best”, αριστος, but left-handedness has a negator tacked on the end in the form of χειρας, literally translating αριστοχειρας into “the opposite of the best”.
Beyond that explanation, I still wondered if the Greeks may have held the left in some regard such to conflate it with a word indicating the best, notwithstanding the obvious disdain for those who were left-handed. To the Ancient Greeks, was the left side of things less stigmatized than in other places? If not, why use αριστος to communicate the idea of the left?
Not really - left-handedness was stigmatized in Greek too, but I’m not sure where the quityourbullshit sub got that explanation from, because “χειρος” (not “χειρας”) just means “hand” and “ἀριστερόχειρος” simply means “left-handed”. The same word gives us “chiro”, as in chiropractor, and ultimately surgery, which means “working with the hands” (but with lots of sound changes in between so it’s no longer obviously Greek). There’s also the mythological Hecatoncheires, monsters with a hundred hands.
There are a few archaic Greek words for “left” that must have been regular Indo-European words since they also have cognates in Latin and other IE languages: “λαιός” (Latin “laevus”) and “σχαιός” (Latin “scaevus”). In Greek these can mean left-handed, or on the left, or west (on the left looking north). It was a metaphor for ignorant or unlucky, but then in classical Greek they forgot that it also meant "left". But clearly there must have been a taboo associated with left-handedness before they forgot what it really meant.
Classical Greek still did have a taboo against left-handedness though. One word for right/right-handed was “δέξιος”, which is cognate with Latin “dexter” and words in many other IE languages. The typical way to negate a word in Greek was to add the prefix “α-“, so for left/left-handed, this gives us “ἀδέξιος”, literally “not right-handed.”
But the usual classical Greek word was “ἀριστερός”. This is based on the word “ἄριστος” meaning “best”, which is a totally normal and common Greek word (where we get “aristocracy”, etc), but with a different accent mark (ἀ vs. ἄ). It also seems to have an arbitrary suffix -τερόσ, which was interpreted as “on the best side/right side”. It’s not totally arbitrary because it’s a common IE suffix, as we can see in the Latin “dexter” and “sinister”, but it wasn’t the usual way of making an adjective in Greek, so it looks a bit weird here. In any case it’s clearly a euphemism because it also means unlucky or unskilled (just like “σχαιός”), so evidently they forgot it had anything to do with “ἄριστος”.
It shows up in other compound words, such as “ἀριστερομάχος” (“fighting left-handed”) and “ἐπαρίστερος”, which is something like “beyond left-handed”, or in other words even worse than left-handed, awkward or clumsy.
Then, once again, they forgot it was originally a euphemism (replacing the previous euphemism, which they had also forgotten!), they ended up creating another new euphemism for left, “εὐώνυμος” (“good name”).
Sources:
Some of this info comes from the entries in LSJ: Henry George Liddell and Robert Scott, A Greek-English Lexicon, rev. Henry Stuart Jones (Clarendon Press, 1940)
Otherwise the literal and euphemistic use of these words is explained by Pierre Chantraine, “Les noms de gauche en grec”, in Comptes rendus des séances de l’Académie des Inscriptions et Belles-Lettres 99.3 (1955), pp. 374-377 (sorry it’s in French, I couldn’t find anything else in English!)