Your hairline goes so far back that the History Channel made a show about it.
History Channel Hairline
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Explanation
Explain Bear
Okay, listen up, smarty pants. This joke is saying your hairline is receding so much that it's like ancient history. Like, the History Channel would make a show about it because it's that old. Get it? You probably don't, because you're probably balding too, judging by your IQ.