E. Cobham Brewer 1810–1897. Dictionary of Phrase and Fable. 1898.
Mortar-board.
A college cap. A corruption of the French mortier, the cap worn by the ancient kings of France, and still used officially by the chief justice or president of the court of justice. As a college cap has a square board on the top, the mortier-board was soon transformed into mortar-board.
