dopey, n.² — Green’s Dictionary of Slang

dopey n.²

[UK, US dial. doup, dowp (first recorded in context of the bottom of an egg), the buttocks]

the buttocks, the rump.

1889–90		Barrère & Leland Dict. of Sl., Jargon and Cant.
1890–1904		Farmer & Henley Sl. and Its Analogues II 309/2: Dopey the podex (old).
1937–84		Partridge DSUE (1984) 332/1: C.18.