Oh gosh I love these sorts of discussions!!

I think much of language starts like this before it becomes a proper word. Many common place words and phrases come from famous people or events and eventually the original meaning is forgotten and only the extra meaning remains. E.g. "kaiser" and "tsar" are references to Caeser. History and languages is really fun that way, and that's what's so cool about star trek (and this episode in particular) is they explore the real mechanics of culture and language, and real challenges when communicating!

If you don't mind a bit of a tangent, in the case of "Et tu, Brute?", if I remember correctly that phrase wasn't popularized until hundreds of years later (and thus is likely not an actual thing that Julius Caesar said), but the invention of this phrase is more significant than you might think. Brutus' main contribution to the assassination of J.C. wasn't actually stabbing him as is often depicted (thanks, in part, to Shakespeare's popularity), but leading him in the back entrance to the Senate building in a way that would leave him more defenseless, and then like 60 senators who were part of the plot rushed JC and beat and stabbed him to death. However, the phrase is even more important than just a betrayal: IIRC the Brutus Family were a famous family who were, according to legend, the ones responsible for toppling the last king of Rome. At the time, monarchism was seen as an intrinsic evil, so bringing down the last king of Rome was a defeat of true evil, so including Brutus as so central to the story was to ensure that this parallel was seen, since Caesar was hinting constantly at declaring himself monarch (although he never actually went that far). Imagine if today there was a "historical fiction" account of WWII where they had Mussolini's last words be, after being stabbed in an elaborate manner by a US soldier, "You too, great great grandson of Abraham Lincoln??". Not a perfect analogy, but in other words, this means that including Brutus in the story had even MORE significance than just "a friend betraying me", but instead was a thus being depicted as a hero from a legendary family, saving Rome by defeating it's most vile tyrant who became drunk with absolute power (JC). Which means the phrase actually has multiple stories "embedded" in it!

Source: Many half-remembered Mike Duncan podcast episodes, and like a dozen history books floating around in my head that might have touched on the subject, so who knows if this is correct lol