It is taken from the Greek letters χ (Ch) and ρ (R) from the word: Χριστος which is Greek for Christ.
You can read more about that on Wikipedia or we did a fantastic lapbook from Hands of a Child that went into all the different symbols/ traditions of Christmas.
Why do I bring this up? We as Christians need to be careful when we start thinking we know everything. Looking at the history or origin of something might give you knowledge of a true meaning rather than relying on a meme on facebook.
Also, we are not supposed to be easily offended. When someone says something like Happy X-Mas to you - maybe try not to correct or take offense. You can easily reply with Merry Christmas! While that is what I choose to say when I'm out and about to strangers, family and friends. I also choose not to be offended by the alternative anymore. Keeping Christ in Christmas is so much more than a holiday greeting. We make a big focus on reading the Christmas story from our bible on Christmas morning and we have a cake for Jesus and sing Happy Birthday. That is really keeping Christ as the focus not being rude to possible unbelievers or perhaps believers that know the bigger meaning behind X-Mas.
One of my husband's favorite verses is this:
Great peace have they which love thy law: and nothing shall offend them. - Psalm 119:165 KJV
This post is part of BenandMe.com's blogging through the alphabet. Click on the picture below for more posts on the letter X.