bongo butt, n. — Green’s Dictionary of Slang

bongo butt n.

[echoic bong!; however note stereotype of the large-buttocked African woman and thus bongo n.¹]

(US) sexually attractive female buttocks, supposedly promoting the desire to be playfully slapped.

1993		[film title] Bongo Butt Babes.
2008		(ref. to 1987) vaultofevil.proboards.com 16 July 🌐 I wasn’t reckoning with the irresistable pull of [...] masked wrestler El Bordah (reluctantly dragging himself away from his copy of Bongo Butt magazine [...] in Charles Burns’ Love in Vein 1987)’ .
2013		Purely 18 #34 24: Equally as impressive is the way her bongo butt bounces in cowgirl.