Green’s Dictionary of Slang

1. (US) an extremely unpleasant person.

1979

Easyriders Dec. 6: A pretty crafty way [...] to get us bumfucks to read your rag cover to cover [HDAS].

2. (US gay) a sexually inadequate partner.

1972

B. Rodgers Queens’ Vernacular 107: Sex partner whose performance was inadequate. [...] bum fuck.

3. anal intercourse.

1989		J. Morton Lowspeak.
1993		T. Jones Pugilist at Rest 119: Neither of us likes to think of these details and instead we make love – bum fucks for bothe of us.