Green’s Dictionary of Slang

(US) the hand, as used in male masturbation.

1992

(con. 1910s) F.M. Davis Livin’ the Blues 41: ‘You might fool these tadpoles,’ he went on waving his hand toward the younger boys [...] ‘but you gotta put somethin’ straighter’n that for an old ace like me. You never had nothin’ but ol’ Minnie Five Fingers!’.