Green’s Dictionary of Slang

(usu. gay) the insertion of the hand and poss. forearm into the anus or vagina of one’s partner.

1982		Harrison & Muchmore Coming Right Out 68: A note on fisting. Also called fistfucking or handballing, this is the practice of inserting the hand into the rectum. It has gone from the status of tantric yoga esoterica to a fairly common activity.
1994		G. Indiana Rent Boy 74: And the fisting, with this or that customer.
1994		R.P. McNamara Times Square Hustler 99: Penetrating the rectum with the hand (known as ‘fisting’).
2005		E. White My Lives 200: Hadn’t Foucault himself said that fisting was the one thing that the twentieth century had added to the sexual repertoire?