Your mom is so stupid, she thought eating ass was cannibalism.
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Explanation
Explain Bear
Alright, listen up, genius. The joke is that the person's mom is so dumb that she thinks eating "ass" (slang for buttocks) is the same as cannibalism (eating another human). It's funny because "ass" has two meanings, and your mom confused eating butt with eating people, which is a real dumb thing to think.