Definition of rusty dusty in English:

rusty dusty

noun

informal
  • A person's buttocks:

    ‘I'll have to whip your rusty dusty’
    • ‘I'm embarrassed to admit that this fine film came and went without my getting up off my rusty dusty to go and see it.’
    buttocks, behind, backside, rear, rear end, seat, haunches, cheeks
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Origin

Late 16th century (in the sense ‘dusty, fusty’): reduplication of rusty. The current transferred use originated in African-American usage in the 1940s.