Green’s Dictionary of Slang

Uncle Thomas n.

[John Thomas n.]

(US) the penis.

[US] in Randolph & Legman Ozark Folksongs and Folklore (1992) I 475: The lady called Casey at half-past four, / When he woke up, she said she wanted more. / So he mounted to the mattress with his organ in his hand, / And he steered his Uncle Thomas to the Promised Land.