What is the X in X-mas?
Old English agen "one's own," literally "possessed by," from Proto-Germanic *aigana- "possessed, owned" (cf. Old Saxon egan, Old Frisian egin, Old Norse eiginn, Dutch eigen, German eigen "own"), from past participle of PIE *aik- "to be master of, possess," source of Old English agan "to have" (see owe).
evolved in early Middle English from Old English geagnian, from root agan "to have, to own" (see owe), and in part from the adjective own (q.v.). It became obsolete after c.1300, but was revived early 17c., in part as a back-formation of owner (mid-14c.), which continued. Related: Owned; owning. To own up "make full confession" is from 1853.