Xmas (n.) Look up Xmas at Dictionary.com
"Christmas," 1551, X'temmas, wherein the X is an abbreviation for Christ in Christmas, English letter X being identical in form (but not sound signification) to Greek chi, the first letter of Greek Christos "Christ" (see Christ). The earlier way to abbreviate the word in English was Xp- or Xr- (corresponding to the "Chr-" in Greek Χριστος), and the form Xres mæsse for "Christmas" appears in the "Anglo-Saxon Chronicle" (c.1100).