Contraction ofva'afa''nculo, wherefa' and'n are contractions offare andin, respectively. Literally,“go do [it] in [the] ass”.
CompareFrenchva te faire enculer,Sicilianvâ fa' 'n culu (orvattilla a fari nculari),Spanishvete a tomar por culo,Portuguesevai tomar no cu.
vaffanculo