What is the X in X-mas?
supreme god of the ancient Greeks, 1706, from Greek, from PIE *dewos- "god" (cf. Latin deus "god," Old Persian daiva- "demon, evil god," Old Church Slavonic deivai, Sanskrit deva-), from root *dyeu- "to gleam, to shine;" also the root of words for "sky" and "day" (see diurnal). The god-sense is originally "shining," but "whether as originally sun-god or as lightener" is not now clear.
The chief of the Greek and Roman gods, who defeated the Titans to assume leadership of the universe. He lived atop Mount Olympus, from which he hurled thunderbolts to announce his anger. Despite his awesome power, he had a weakness for mortal women. (See Leda and the swan.)