Green’s Dictionary of Slang

buttfuck v.

also b.f., butt-bang
[butt n.1 (1a) + fuck v. (1)/bang v.1 (2a)]

1. [1960s+] (US) to subject to anal intercourse; thus buttfucking n. and adj.

2. [1970s+] in fig. sense, to treat unfairly, to cheat, to deceive; thus buttfucking n.

3. [1980s] (US) to injure.