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

bustle n.²

[SE bustle, a ‘dress-improver’; i.e. a small pad or wire framework that accentuates the back of the dress]

(US) the buttocks, the posterior.

1839		‘Mrs Crow’s Arrival’ in Jim Crow’s Song-Book 10: Kase me got sich a pretty slender waist, / And lots ob nat’ral bustle.
1928		Hecht & MacArthur Front Page Act III: What a fine horse’s bustle I turned out to be!
1942		N. Algren Never Come Morning (1988) 205: Ben hustlin’ my bustle between here ’n Baton Rouge goin’ on fourteen years now.