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Pain in the bum is like getting kicked by a donkey wearing boots. It’s British slang for being annoyed by someone’s butt, and it’s not as gross as pain in the ass, but still pretty bad.
My brother is pain in the bum because he won't stop poking me.
My teacher says I'm pain in the bum for talking too much.
My dog is pain in the bum because he keeps sitting on my feet.