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.