How do you say asshat in Latin?

Culipileus is what I'd say. It doesn't appear in any classical writing, needless to say, but I've coined it as a compound of culus (ass) and pileus (hat). Or if you want to use a full sentence instead, you could say Culum pro pileo habes ('You have (your) ass for a hat'.)

Hoc utilem esse spero! (Hope it's useful!)