PRONUNCIATION: krsms, ksms
ETYMOLOGY: From X, the Greek letter chi, first letter of Greek Khrstos, Christ. See Christ.
USAGE NOTE: Xmas has been used for hundreds of years in religious writing, where the X represents a Greek chi, the first letter of , Christ. In this use it is parallel to other forms like Xtian, Christian. But people unaware of the Greek origin of this X often mistakenly interpret Xmas as an informal shortening pronounced (ksms). Many therefore frown upon the term Xmas because it seems to them a commercial convenience that omits Christ from Christmas.
From the American Heritage Dictionary