Green’s Dictionary of Slang

boy, the n.¹

the penis.

[a.1674		Rochester ‘The Argument’ in Poems on Sev. Occasions (1685) 34: How Tall-Boy, Kill-Prick, Suck-Prick did contend For Bridegroom Dildo [...] Gigantick Tall-Boy, famed in the West [...] Tall-Boy with one Hand held Jove’s Prize].
1838		‘Rory O’More Had A Hell Of A Bore’ in Rambler’s Flash Songster 12: My darlings, said Rory, come one at a time. / And I’m the boy your touch holes to prime.
1972		George Anthony ‘Cock Stiff and hard’ 🎵 My boy is long and stout.
1992		P. McCabe Butcher Boy (1993) 138: I’m the man that would slip the boy in there double quick!