Green’s Dictionary of Slang

(US) the penis.

1934		in Randolph & Legman Ozark Folksongs and Folklore (1992) II 667: He layed his jong out on the bar, / By God, it stretched from thar to thar!
1992		(con. 1920–57) Randolph & Legman Ozark Folksongs and Folklore II 787: Other common names for the male organ are [...] jock, john, jong.