Green’s Dictionary of Slang

(US) the penis.

1948		in Randolph & Legman Ozark Folksongs and Folklore (1992) II 656: He wrapped his thumb around his wong.
1972		B. Rodgers Queens’ Vernacular 49: the penis [...] Mr. Wong. [Ibid.] 215: wong 1. the penis.
1974		J. Lahr Hot to Trot 21: Does it feel like a wong?
1988		in H. Max Gay (S)language.