Green’s Dictionary of Slang

(Polari) the (erect) penis.

1968		K. Williams 12 May Diaries (1994) 326: Tom described having it away and I got half a colin, Clive said ‘Well get it out then—come on, show us’ but I laughed it off.
2019		P. Baker Fabulosa 290/2: colin 1. an erect penis.
2023		R. Milward Man-Eating Typewriter 33: A fortuni chiselled husband who’d stuck his colin in the most meese greaser on your strasse.
2023		R. Milward Man-Eating Typewriter 52: I’d been half expecting to get a colin myself, but the manky spectacle rapidly threw acqua on my arousal.