Green’s Dictionary of Slang

1. any variation on ‘straight’ heterosexual intercourse, esp. anal intercourse.

1969		C. Brown Life and Loves of Mr Jiveass Nigger (2008) 149: Hey, that cat’s the next best freak-fuck in town, next to the Greeks.
1980		E. Folb Runnin’ Down Some Lines.
2000		Salon.com 19 Feb. 🌐 Plus, she says, many disabled women are caught in a Catch-22, since many of the few men who aren’t turned off by the chair are just after ‘a freak fuck’.
2000		R.J. Arobateau Street of Dreams 148: She’s so much an actress I was fooled by the lezzie freak fuck show last week.

2. (orig. US black) a client who demands unusual or poss. physically dangerous services from a prostitute.

2006

in M. Christian Transgender Erotica 173: I got a call as a player for a ball-busting freak fuck that ended when someone died.