The thicker or heavier end of something, or colloquially, the human buttocks. Can also mean targets for archery or rifle practice.
From Middle English 'butte' meaning 'thick end, target mound', from Old French 'but' meaning 'goal, target'. The anatomical sense developed as slang in the early 20th century from the idea of the 'end' of the body.
The archery meaning is the original - medieval archers practiced by shooting at earth mounds called 'butts'. The phrase 'butt of the joke' comes from this target meaning, not the anatomical one, referring to someone being the target of humor.
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