This abbreviation for Christmas is of Greek origin. The word for Christ in Greek is Xristos. During the 16th century, Europeans began using the first initial of Christ's name, "X" in place of the word Christ in Christmas as a shorthand form of the word. Although the early Christians understood that X stood for Christ's name, later Christians who did not understand the Greek language mistook "Xmas" as a sign of disrespect.
(courtesy of The Symbols and Traditions of Christmas)
so that's really neat. and no body even knows that. did you? of course you didn't.