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bilky asko5,796 posts since 9 Sep 2006
Tyne Tees Look North (North East)
His name was Jesus Christ, not Jesus X! Laughing


Wiktionary gives a nice etymology:

Attested in this form since 1755, and preceded by earlier forms such as X'temmas (1551) and Old English Χp̃es mæssa (1100), from Christmas, replacing Christ by the abbreviation X, from Ancient Greek Χ ‎(Kh, “(letter chi)”), from Χριστός ‎(Khristós, “Christ”). Surface analysis X ‎(“Christ”) +‎ -mas ‎(“holiday”). In popular use since late 19th century.