Green’s Dictionary of Slang

(US) the buttocks, esp. with the implication that someone has been sitting around doing nothing; thus they are rusty and dusty from lack of movement.

1942		Count Basie Orchestra ‘Harvard Blues’ 🎵 Mama, get up of your big fat rusty dusty.
1953		‘William Lee’ Junkie (1966) 40: Get up, get up woman, off your big, fat rusty-dusty.
1956		‘Ed Lacy’ Men from the Boys (1967) 109: Now drag your rusty up here on the double.
1973		L. Bangs in Psychotic Reactions (1988) 121: Get off your big fat rusty dusty and man them barricades with me!
1979		E. Torres After Hours 74: Not with that rusty-dusty she sits on.
2000		Ebonics Primer at www.dolemite.com 🌐 rusty-dusty Definition: one’s ass Example: Get off your ole’ rusty-dusty and bring me back some cognac biatch, don’t be trippin’ neither!