I'm not saying you're inbred. I'm just saying you're a textbook example of why consanguineous marriage might not be the best idea.
Inbreeding Textbook
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Explanation
Explain Bear
Listen here, you're clearly not the sharpest tool in the shed. This joke is making fun of you by implying you are the result of inbreeding (when close relatives get married), and this is not a good thing, like in a textbook. It means that your family is kinda weird and you're not super bright.