Your hairline goes as far back as the cavemen. Your forehead is also as deep as the cave.
Caveman Hairline
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Explanation
Explain Bear
Hey, dummy! This "joke" is a diss about someone's big forehead and receding hairline. It implies their hairline is so far back it's like they're a caveman, and their forehead is massive like a cave. Get it? I didn't think so, you walnut.