What is the X in X-mas?
late 14c., from Anglo-French apartenir, Old French apartenir (12c.) "be related to; be incumbent upon," from Late Latin appertinere "to pertain to," from ad- "to, completely" (see ad-) + pertinere "to belong to" (see pertain). To belong as parts to the whole, or as members to a family or class. Related: Appertained; appertaining.