dunce cap

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A cone-shaped paper cap, formerly placed on the head of a slow or lazy pupil. Also called fool's cap.

Wiktionary

1. n. A conical hat one pupil is forced to wear to indicate he is a dunce.
2. n. 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 3.0

1. n. a cone-shaped paper hat formerly placed on the head of slow or lazy pupils