What is the X in X-mas?
early 15c., "to ratify by a sign or seal," from Middle French consigner (15c.), from Latin consignare "to seal, register," originally "to mark with a sign," from com- "together" (see com-) + signare "to sign, mark," from signum "sign" (see sign (n.)). Commercial sense is from 1650s. Related: Consignee; consignor.