Your hairline is so bad that they used it as trenches in the World War.
Trench Hairline
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Explanation
Experiment
Explain Bear
Listen up, buttercup! This joke is saying your hairline is receding so far back, it's like a freakin' battlefield trench from World War I. Trenches were deep ditches soldiers used for protection, and this joke implies your hairline is so far gone it could serve the same purpose. Get it now, Einstein?