late 14c., morteise, "hole in which something is fitted" (originally of the hole in which Christ's cross was inserted); mid-15c. in the carpentry sense "hollow or groove cut in a piece of wood into which a corresponding projection (called a tenon) is fitted to form a joint;" from Old French mortaise (13c.), which is of uncertain origin. Possibly from Arabic murtazz "fastened," past participle of razza "cut a mortise in." Compare Spanish mortaja.