I noticed an online acquaintance the other day becoming extremely agitated that someone had referred to Christmas using the colloquialism Xmas. She felt that this was insulting, and offensive in the extreme. What she didn't realize was that Xmas as a shortened form for Christmas has a venerable (and solidly Christian) history.
The word Christmas is a compound of Christ + mass; we see it first in Old English in the form Cristes mÃ¦sse in 1038, according to the OED. The Old English form eventually evolved to the Middle English Christemasse. The word Christ is derived from the Greek word Christos, meaning “anointed,” a literal translation of the Hebrew cognate of messiah. Mass, as in the Christian ritual, derives from Middle English, fromOld English, fromVulgar Latin, fromLate Latin, fromLatin, feminine past participle of, to send away, dismiss.]
The X of Xmas is a shorthand way to refer to the name of Christ. In Greek, the language of the New Testament, the Greek letter Chi, written as X (the ancestor of the English letter X) is the first letter of Christ's name. X has been used as an abbreviation for Christ since at least the early 1500s. Earlier, and closely related abbreviations include Xp and Xr, from the Chi, the Rho and the Iota (our letter I/i), the Greek letters that spell the Chr, the first three letters of Christ.
In medieval manuscripts the Chi Rho page is typically a very highly ornamental page in from the beginning of the book of Matthew. The name is because the text is about the birth of Christ from the verse from Matthew 1:18 that in English in the 1611 version begins “Now the birth of Jesus Christ was on this wise: When as his mother Mary was espoused to Joseph, before they came together, she was found with child of the Holy Ghost.” The Latin text, the one used most often in medieval manuscripts, begins “XPI autem generatio . . .” The most famous page in the Book of Kells is the Chi Rho page.
An entry in the Anglo Saxon Chronicle c. 1100 uses the shorthand of Xres masesse for Christmas; an abbreviation like Xmas is not so heretical, after all, and in truth, is quite traditional.