Your hairline goes so far back, we learned about it in history class.
Ancient Hairline
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Explain Bear
Listen up, numbskull. This joke is saying the person's hairline is receding so far back that it's ancient, like something you'd study in history. It's funny because nobody wants to be told they're balding, and comparing it to something old makes it extra insulting. Plus, you probably don't know anything about history, so it's a double whammy!