What is the X in X-mas?
Old English gyrdan "put a belt or girdle around; encircle, surround; invest with attributes," from Proto-Germanic *gurthjanan (cf. Old Norse gyrða, Old Saxon gurdian, Old Frisian gerda, Dutch gorden, Old High German gurtan, German gürten). Related to Old English geard "hedge, enclosure" (see yard (n.1)). Related: Girded; girding.
Throughout its whole history the English word is chiefly employed in rhetorical language, in many instances with more or less direct allusion to biblical passages. [OED]