Green’s Dictionary of Slang

dinky n.⁴

the penis.

1972		B. Rodgers Queens’ Vernacular 64: dinky 1. the cock when shriveled up by the cold.
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.