Green’s Dictionary of Slang

(US) an act of sexual intercourse engaged in out of pity; thus also v.

1978		E. Kazan Acts of Love 114: You give him one more night, what men call a mercy fuck.
1987		C. Hiaasen Double Whammy (1990) 204: I just want you to know, it wouldn’t have been a mercy fuck. It would have been the real thing.
1993		R. Shell Iced 146: I took Maggie back to my place and gave her a mercy fuck.
1997		(con. 1970s) G. Pelecanos King Suckerman (1998) 88: Nothing but mercy-pussy for young Russell.
2001		‘Mr. Kiosetic’ Tommy and Shelly 🌐 The ink on Joe and Laura’s divorce hadn’t even dried before they were at it, even if it was for one last time. A mercy fuck, ‘for old times sake,’ whatever you wanted to call it.