colleen bawn, n. — Green’s Dictionary of Slang

colleen bawn n.

also colleen

[rhy. sl. = horn n.² (1b); ult. the anglicized version of Irish cailín bán, the white or fair woman; Colleen Bawn was the heroine of the opera ‘The Lily of Killarney’ (Benedict/Oxenford & Dion Boucicault), first produced Feb. 1862 at the Royal English Opera, Covent Garden, London]

an erection.

1960		J. Franklyn Dict. of Rhy. Sl.
1969		S.T. Kendall Up the Frog 64: Colleen Bawn (Colleen).
1978		Maledicta II:1+2 (Summer/Winter) 116: The concentration in that article on proper names produced [...] play titles (as Colleen Bawn = ‘horn’, that is erection also Marquis of Lorne).
1980		Maledicta IV:2 (Winter) 192: Single inversion is seen […] to a certain extent in rhyming slang, where the elision of the rhymed word is common. For example, Colleen = Colleen Bawn (= rhyming slang on born), as is Marquess = Marquess of Lorne.