Green’s Dictionary of Slang

(US) to have sexual intercourse with someone out of pity; thus mercy-fucking n.

1968		P. Newman Playboy July n.p.: There was sport fucking. There was mercy fucking, which would be reserved for spinsters and librarians [R].
1980		M. Gordon Company of Women 141: ‘Who’s this one?’ she heard someone say. ‘One of Robert’s rescuees. He believes in mercy-fucking.’.
1989		A. Vachss Hard Candy (1990) 118: She mercy-fucked some poor shlub and he bought her a pearl necklace.
2006		G. Nicholson Sex Collectors 217: We mercy-fucked him and gave him fifteen dollars.
2008		A. Wheatle Dirty South 152: He was mercy-wokked by Shyanne Moore after a burning session at Blackie Norton’s gates.