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When you butt heads with someone on purpose or by accident. Like a butt version of a kiss, but way more awkward and smelly.
My cousin and I bumped butts at the buffet. It was like a slap fight, but with ass.
I accidentally bumped butts with my boss. He looked like he wanted to kill me.
My friend and I bumped butts during a dance-off. We both fell over like farts in a hallway.