Green’s Dictionary of Slang

wong n.1

also Mr Wong
[var. on wang n.2 (1)]

(US) the penis.

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