What is the X in X-mas?
1715, "style of dress," an art term, from French costume (17c.), from Italian costume "fashion, habit," from Latin consuetudinem (nominative consuetudo) "custom, habit, usage." Essentially the same word as custom but arriving by a different etymology. From "customary clothes of the particular period in which the scene is laid," meaning broadened by 1818 to "any defined mode of dress." Costume jewelry is first attested 1933.
1823, from costume (n.). Related: Costumed; costuming.