Your hairline goes so far back, your forehead got a six pack.
Receding Hairline
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Explanation
Explain Bear
Listen up, chucklehead! The joke is that the person's hairline is receding so far back that their forehead is now so big it's got abs, like a six-pack. It's like saying your forehead is working out, which is dumb because foreheads don't do that. Plus, I heard you have a five-head!