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

dopey n.²

[ety. unknown; ? link to SE didapper/dydapper (see prev.)]

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.