Green’s Dictionary of Slang

the manual stimulation of the female genitals or of the anus (usu. male).

1884		Randiana 49: The easy transition from a kiss to a feel, from a feel to a finger frig, and eventually [...] to a gentle insertion of the jock.
1971		E.E. Landy Underground Dict. (1972).
1974		Adamson & Hanford Zimmer’s Essay 75: You know what you are bloody good for? A finger-fuck! That’s all!
1999		Gaymart.com Queer Sl. in the Gay 90s 🌐 Finger Fuck – The insertion of all or most of the hand into the vagina or the anus.
2003		K. Cage Gayle 70/1: finger fuck n. the insertion of the middle finger into the anus to stimulate the rectum and prostate gland.
1987		personal ad, adult bookstore Murray & Murrell Lang. Sadomasochism (1989) 69: Fingerfucking, fistfucking, greek, french, dildoes, enemas, all manner of insertion.