An archaic term for a large apple, or a derogatory medieval term for a person's head.
Middle English, possibly from Old French coste (rib) because large apples were ribbed, or related to 'coster' (apple seller); also used as crude slang for head in medieval texts.
Medieval insults got creative—calling someone a 'costard' (apple head) was basically their version of modern trash talk, and the word stuck around long enough that Shakespeare used it in his plays.
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