Green’s Dictionary of Slang

(US) the posterior, the buttocks.

1934		H. Roth Call It Sleep (1977) 344: I’ll hol’ yuh if yer goin’ on yer – you know – oh boy!
1935		(con. 1920s) J.T. Farrell Judgement Day in Studs Lonigan (1936) 545: He must do all the work, while Andy sits on his you-know.