What is the X in X-mas?
Old English unbindan, "to free from binding," from un- (2) + bind (v.). Cf. German entbinden, Dutch ontbinden. Literal and figurative senses both present in Old English.
Suæ huæt ðu unbindes ofer eorðu bið unbunden in heofnum. [Lindisfarne Gospels, Matt. xvi:19]