mortarboard (n.)

also mortar-board, 1823, "square board used by masons to hold mortar for plastering," from mortar (n.1) + board (n.1). By 1854 in reference to the academic cap, probably so called because it resembles the mason's board. Earlier it was called a mortar cap (1680s) or simply morter (c. 1600), from French mortier.