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The mark of ignorance. Before more enlightened pedagogical theories came into practice, students who were considered slow learners or even completely incapable of learning were made to sit on a high stool in a classroom corner while wearing a tall, conical paper cap with the word “dunce” on it. The rationale was that such embarrassment would encourage the child to try harder and do better (perhaps as a traumatized juvenile delinquent). Such headgear has gone the way of the three Rs taught to the tune of a hickory stick, but as a phrase, people who make a mistake then smack themselves up the side of the head while uttering a Homer Simpson–like “doh!” might then ask rhetorically “Where'd I leave that dunce cap?” “Dunce” has an interesting derivation. It comes from the medieval philosopher John Duns Scotius, whose ideas were considered foolish. He and his followers wore conical hats, the top meant to point toward higher wisdom. The similarity to dunce caps is inescapable.
See also: cap