What is the X in X-mas?
1540s, from French inciser (15c.), from Old French enciser (12c.), from Latin incisus, past participle of incidere "to cut into, cut through" (see incision). Related: Incised; incising.
incise in·cise (ĭn-sīz')v. in·cised, in·cis·ing, in·cis·es To cut into with a sharp instrument.