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.
mid-15c., "cut or make a mortise in," from mortise (n.). From 1540s as "join by tenon and mortise." Related: Mortised; mortising.
also dove-tail, 1580s, in carpentry, "tenon cut in the form of a reverse wedge," the strongest of all fastenings, from dove (n.) + tail (n.). So called from resemblance of shape in the tenon or mortise of the joints to that of the bird's tail display. As a verb, "to unite by dovetail tenons," 1650s; figuratively "unite closely, as if by dovetails." Related: Dovetailed.