Sicilian-American evolution of the Italian expression "Fa' un culo," which literally means "Do an ass." Many guidos and other not-really-Italian-but-really-wish-I-weres use this a lot. Also, some REAL Italian-Americans too :D
Guido: Eyyyyy Fangul! Dontcha kno who ah AM?!?! IM DA REAL ITALIANO BABY JERSEY SHORR
Italian: Cosa??? Torni al tuo paesi del cazzo.
A gesture, Italian in origin, that originally meant "You are dead to me". It has been construed to it's modern day bastardization meaning "Fuck you". However, it is still used in it's original sense by many Italian families.