Green’s Dictionary of Slang

accordion n.

[the physical resemblance to the instrument]

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

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