Green’s Dictionary of Slang

(Polari) the (erect) penis.

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.