Your hairline goes so far back, I remember seeing it in the Stone Age.
Stone Age Hairline
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Explanation
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Explain Bear
Alright, listen up, buttercup. This joke is saying your hairline is receding so far back on your head that it's like it's been around since the Stone Age, which was, like, a really, really long time ago. It's a funny way of saying you're balding, ya dingus.