Some Greek insults (the list is, of course, in no way even close to complete):
The main word that means a-hole literally translates to "masturbator".
A word that's stronger than a-hole literally translates is a dirty word version of "period pad".
Want to rudely tell someone that "hah, you can't touch me, you can do nothing to me!" when they threaten you to do something and you know they can't touch you? You can tell them that they're "gonna fart you a stockyard of balls". Why? I have no idea, lol!
Words and phrases that can be used in Greek to call someone stupid and synonyms (I went with the more imaginative ones here): Brick, IQ of a chicory, idiot with a helmet, rush (the plant), purple amaranth (it's just 2 syllables in Greek), booby (the bird), rooster-brained, okra, stucco, untinned bin (where the tin has been removed and is in need of repair), log, ox, pickaxe, adze, bran-brained, a-hole bran, bouzouki brain, davul, baglama, recorder, UFO, cork, halvah, and oh so many others...
Unfortunately there were a number that I couldn't include here because there is just no translation for that, and for most people here that would just be a group of syllables and their inventiveness or funniness would not be appreciated.
Not an insult, but here's a bonus because it's just so random: A way to tell someone in Greek that you're gonna beat them up is the Greek translation of "you will eat wood".