Why is Christmas abbreviated Xmas?

Hey there, wisdom scout:

While making our list of family items to do this week, I used the abbreviation "Xmas" to indicate shopping for the holiday. My 12-year-old son asked me why it was abbreviated as "Xmas," and in my wisest voice, I muttered, "I don't know." Do you know the answer to this so I can get my title as "knower of everything" back?

-- kiyotei, the Net

The kid's 12? Even if you get your title back, it's maybe good for only a year or two, at best. After that it doesn't matter how smart you are. You're gonna just be one more old fogey. So I guess we better get on this one fast to maximize your shelf life.

Forget any stories you may have heard about the word Xmas being some smart aleck way of taking "Christ out of Christmas." Some people consider it disrespectful. Well, no way. X is the Roman alphabet equivalent of the Greek letter chi, the first letter in "Christ." It was a common ancient abbreviation when writing "Christian" (Xian) and similar words. So there you go, Guru Dad. Enjoy it while ya got it.