We like to pepper modern English, in particular our scientific and technical terms, with Latin/Ancient Greek words and phrases, and it recently occurred to me to wonder whether any of them would make sense to an Ancient Roman or Greek native speaker. Would species names like Homo sapiens or Felis catus make sense? If I called something ad hoc, sui generis, ad hominem or ex nihilo, would a native speaker understand them to have the same meaning I would? If I told an ancient Greek physician I had hepatitis or schizophrenia, would he have the slightest chance of working out what I was talking about? Would any of it sound like normal speech or would it be like going to a doctor today and complaining of gutsouching, or accusing someone I'm having an argument with of managainstness?
(This question inspired by own recent bout of pericoronitis, which apparently is a combination of Greek and Latin elements and thus would probably sound extremely silly at best.)
ad hoc, sui generis, ad hominem or ex nihilo,
None of these terms appear in the classical texts (at least according to my Chambers-Murray); they appear to be post-medieval coinings.
I suspect that a Roman of the classical period would have no problem at all understanding ex nihilo. It literally means "out of nothing" and is really no more complicated than that. And that's also what we mean when we say it in English.
Sui generis would probably not be much of a problem, with the kind of context that would occur in a conversation, but it would require some unpacking. It literally means "of its own type" in Latin; when we use it in English we tend to mean "uniquely or distinctively of its own type, in a way that makes it different and in some way not comparable with other types." The modern connotations would not be clear from simply saying "sui generis", but in a conversation like:
All birds can fly.
The dodo is a bird, and it can't fly.
Dodos are sui generis.
The meaning would probably come through.
Ad hominem or argumentum ad hominem would not be clear to most people.
It's possible that someone versed in philosophy might recognize "argumentum ad hominem" as the same "against the questioner" fallacy described by Aristotle in Topics. But that would require an implausible level of insight, I think.
From Topics, Book 10:
There are four possible ways of preventing a man from working his argument to a conclusion. It can be done either by demolishing the point on which the falsehood that comes about depends, or by stating an objection directed against the questioner: for often when a solution has not as a matter of fact been brought, yet the questioner is rendered thereby unable to pursue the argument any farther. Thirdly, one may object to the questions asked: for it may happen that what the questioner wants does not follow from the questions he has asked because he has asked them badly, whereas if something additional be granted the conclusion comes about. If, then, the questioner be unable to pursue his argument farther, the objection would properly be directed against the questioner; if he can do so, then it would be against his questions. The fourth and worst kind of objection is that which is directed to the time allowed for discussion: for some people bring objections of a kind which would take longer to answer than the length of the discussion in hand.
There are then, as we said, four ways of making objections: but of them the first alone is a solution: the others are just hindrances and stumbling-blocks to prevent the conclusions.
I don't think anyone would get ad hoc without explaining precisely what is meant, any more than an English speaker would understand what you meant if you said "Well, that's kind of to this reasoning". "Hoc" is the accusative form of "hic", meaning this; "ad" is a preposition that takes the accusative, so hic becomes hoc following ad. (In the same way that the irregular noun "homo" becomes accusative "hominem" after ad.
I think species names would mostly just be confusing. It's not at all apparent that "wise man" (homo sapiens) actually refers to all modern humans as a category.
I'm less sure about Greek, but I think that most diseases would be pretty cryptic, unless they are really clear or were used in Greek. "Arthritis" was the actual Greek word for arthritis; I think it's why -itis is generally used to mean inflammation in modern disease names. Glaucoma was the actual Greek term for, we think, cataracts. Agoraphobia (=fear of the marketplace) might come across as well.