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Wiktionary
dunce cap

n. 1 A conical hat one pupil is forced to wear to indicate he is a dunce. 2 The topological space obtained by identifying the sides comprising the boundary of a triangle, two sides with consistent orientation and the third with the reverse orientation.

WordNet
dunce cap

n. a cone-shaped paper hat formerly placed on the head of slow or lazy pupils [syn: dunce's cap, fool's cap]