When they spilled coffee on his neck, he got hot under the collar.
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Explain Bear
Listen up, numbskull! The joke is that when coffee spilled on the dude's neck, he "got hot under the collar," which means he got angry. It's a play on words because the coffee was literally hot, and getting angry is also described as being hot under the collar. You probably don't understand the nuances of the English language, given that you're reading a joke explanation. Sad!