This was a really tough letter for me. I've racked my brain (not a pretty sight) and finally came up with only one possibility. The word Xmas and where it came from.
It goes back a lot farther than I realized, for at least 1,000 years, long before our modern Xmas was used. X is a symbol for "Christ" which was often written "Xp" or Xt", referenced in the Anglo-Saxon Chronicle as far back as 1021. The X and P were the uppercase forms of the Greek letters x (Ch) and p (R) used in ancient abbreviations for the Greek word Christ. Many Eastern Orthodox Icons still use them in showing Jesus Christ. The labarum is a symbol often used to represent Christ in Catholic, Protestant, and Orthodox Christian Churches. (I can't write those letters so here's a picture.)
The labarum, often called the Chi-Rho, is a
Christian symbol representing Christ.
The Oxford English Dictionary shows "X-" or "Xp-" for Christ as early as1485. There are other uses of "(Xt)" for Chris(t)-" too.
I learned something here. Perhaps you already knew. Anything you'd like to add.