Green’s Dictionary of Slang

(US campus/gay) an uncircumcised penis.

1972		B. Rodgers Queens’ Vernacular 203: wear a turtle-neck sweater to be uncircumcised.
1983		Eble Campus Sl. Nov. 5: winter wear – the foreskin of an uncircumsized male. Also turtleneck.
1988		H. Max Gay (S)language 43: Turtleneck sweater — foreskin.
2000		P. Beatty Tuff 84: The last thing Spencer felt was Epstein stencilling a very crooked line around what he called the ‘turtleneck’ of his penis.