Green’s Dictionary of Slang

the use of one or more fingers to stimulate the clitoris or penetrate the vagina.

1965		H. Huncke in Huncke’s Journal (1998) 8: Licking — eating — jerking off — and finger fucking.
1985		‘Thommo’ Dict. Aus. Swearing & Sex Sayings 46: FINGER FUCKING — Female masturbation.
1988		P. Califia Macho Sluts 29: I love french kissing and finger fucking.