Green’s Dictionary of Slang

dinky n.⁴

the penis.

1935		A.W. Read N.W. Amer. Folk Epigraphy 49: The penis [...] A child’s term is dinkie.
1970		V. Bosch Sexual Dimensions 22/1: ‘Jack’s little dinky just barely broke my maidenhead the night we married’.
1972		B. Rodgers Queens’ Vernacular 64: dinky 1. the cock when shriveled up by the cold.
1984		‘John Coriolan’ Smile of Eros 69: ‘The meeting you’ll be invited to is kind of like a club—no dinkies invited, nobody that swings less than eight inches’ [Simes:DLSS].
1985		‘Paul Radley’ Good mates! 279: ‘This way you either stay here or go get your dinky shot off for mourning womankind’ [Simes:DLSS].
1990		P. Davies All Played Out 266: I wouldn’t know if Diego’s dinky is really that ginormous.
1999		‘COMMONWEALTH vs. CLINT C., a juvenile’ on Social Law Library 🌐 The victim stated that the juvenile came into the room, opened his pants, exposed his penis, and placed it in her mouth, that ‘he made me suck his dinky,’ and that she had to stroke his penis with her hands while she was doing this.

In phrases

diddle the dinky (v.) [diddle v.¹ (2)]

to masturbate.

2003

Number One Adult Sexual Health Terms Advisor 🌐 Masturbation Slang Male Terms: [...] diddling the dinky.