Green’s Dictionary of Slang

(US) the penis.

1905–07		‘Old Gingerbread’ in Bawdy N.Y. State MS. n.p.: One hand grasped a knife, and the other my old ding dong, / And off came old ginger bread nine inches long.
1944		PADS 2.29: Ding-dong [...] the penis.
1958		Southern & Hoffenberg Candy (1970) 153: I dug the de-frocked ding-dong of his which I’ve no doubt you know only too well!
1967		Velvet Undergound ‘Sister Ray’ 🎵 She’s busy sucking on my ding-dong.
1978		L. Kramer Faggots 233: Naked sprites, their ding-dongs bouncing up and down.
1985		O. Hawkins Chili 42: Gino had [...] a small ding dong.
2004		N. Kelley ‘The Code’ in Brooklyn Noir 187: ‘King Kong with a powerful ding-dong!!!’ he roared.
2024		J. Meades Empty Wigs (t/s) 316: She’s busy sucking on my dingdong.