What is the X in X-mas?
Old English ebba "ebb, low tide," perhaps from Proto-Germanic *abjon (cf. Old Frisian ebba, Old Saxon ebbiunga, Middle Dutch ebbe, Dutch eb, German Ebbe), from *ab-, from PIE root *apo- "off, away" (see apo-). Figurative sense of "decline, decay" is c.1400.
Old English ebbian, from the root of ebb (n.). Related: Ebbed; ebbing.