Green’s Dictionary of Slang

(US gay) a penis that is substantially larger than expected when erect.

[1956		J. Stearn Sisters of the Night 103: She’s a lot of fun and she plays the accordion].
1972		B. Rodgers Queens’ Vernacular 51: accordion (late ’60s) an unexcited unexciting cock which expands to a much greater length when aroused.
1989–2003		R.O. Scott Gay Sl. Dict. 🌐 accordion: [late 60s] a penis which becomes dramatically longer in length when aroused.