dunce cap
US /ˈdʌns kæp/
UK /ˈdʌns kæp/
1.
a conical hat, typically made of paper and inscribed with the word 'dunce', formerly worn as a punishment by children who had been slow to learn or misbehaved in school
:
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In old cartoons, disobedient students were often made to wear a dunce cap.
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The teacher threatened to make him wear a dunce cap if he didn't pay attention.