What is the X in X-mas?
1550s, from Medieval Latin incineratus "reduced to ashes," pp. of incinerare, from Latin in- "into" (see in- (2)) + cinis (genitive cineris) "ashes," from PIE root *keni- "dust, ashes" (cf. Greek konis "dust"). Used earlier in English as a past participle adjective meaning "reduced to ashes" (early 15c.). Related: Incinerated; incinerating.