Yo mama's ass is mad crusty.
Crusty Ass
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Explanation
Experiment
Explain Bear
Okay, listen up, you dunce! This "yo mama" gag is like saying someone's mom has a butt so crusty it's beyond help. It's an insult, see? It's funny because you, the joke receiver, are probably as crusty as that imaginary butt. Get it now, genius?