My grandpa and your hairline go way back.
Way Back Hairline
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Explanation
Experiment
Explain Bear
Alright, listen up, buttercup. This joke is saying that the person's hairline is so far back on their head, it's like it's been receding since the time of the speaker's grandpa. Get it? It's an insult disguised as a history lesson! Bet you didn't see that one coming, did you?