Green’s Dictionary of Slang

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

1972		B. Rodgers Queens’ Vernacular 51: accordion (late ’60s) an unexcited unexciting cock which expands to a much greater length when aroused. Syn. surprise package.
1989–2003		R.O. Scott Gay Sl. Dict. 🌐 accordion: [late 60s] a penis which becomes dramatically longer in length when aroused. Synonyms: hidden magic; surprise package.