David Jacobson of Trivworks says that the “X” in “XO” is the letter that stands for the kiss, and cites several theories as to why that is. “The prevailing theory for how this came to be so prevalent, particularly at the end of written correspondence, is that during Medieval times, many people didn’t know how to read or write,” he says. “Rather, when signing important documents, they would simply draw or imprint an ‘X,’ an abbreviation for the word ‘Christ,’ which they would then kiss, out of reverence.”
Thus, over time, the “X” came to be associated with the act of kissing, itself. That is why it is believed to still be used in this fashion today.
Check out the full story from The Healthy, HERE.