In the scene above (and according to the rough translations over at Reddit), Giamatti’s character curses Clancy Brown’s Jock Jeffcoat in the most demeaning ways — from spitting out words like, “Look at you, you stupid ball-licking dumbass, even dogs don’t like you,” to telling him, “You’re so full of sh*t that if you step in it, you’re a perfect match.”
Giamatti doesn’t actually speak Italian — as he tells James Corden and Michael Sheen in the video below — but he said they got an Italian fan who gave them a bunch of “old school Roman slang” to put into his monologue. You probably know what “vaffanculo” means, at least.