A small square board with a handle beneath, for holding mortar; a hawk; used by masons to hold or carry mortar. [1913 Webster]
An academic cap topped by broad, projecting, stiff and flat square top, with a tassel attached to the top and hanging down. It was once worn by students in some colleges, but is now worn usually only at graduation ceremonies. [1913 Webster]
mortarboard, n.
1 an academic cap with a stiff flat square top.
2 a flat board with a handle on the under-surface, for holding mortar in bricklaying etc.