Green’s Dictionary of Slang

(US) the hand, as used in male masturbation; thus married to mary fist, addicted to masturbation.

1950		Goldin et al. DAUL 136/2: Married to Mary Fist. (Coarse prison humor) In prison; hence, by implication, addicted to the practice of masturbation.
1960		(con. 1940s) D. MacCuish Do Not Go Gentle (1962) 124: Bet those women of yours get real jealous of Mary Five-Fingers, eh?
1972		B. Rodgers Queens’ Vernacular 158: If each night becomes jack night exclusively, the man is brushed off as being married to Mary Fist (enslaved to autoeroticism).
1980		Maledicta IV:2 (Winter) 232: Mary Ellen, from an old Music Hall song has in the UK picked up connotations of grope = covert feel.
1986		R. Campbell In La-La Land We Trust (1999) 145: I can understand some gazoony without a woman, or with the wrong woman, watching crap like this and having it off with Mary Fist.