Well, the X in Xmas is from the first Greek letter of the title Khristos 'anointed one'. Chi rho (i.e., XP) is another abbreviation, and a longer one is JHC XPC (from the first two letter and the final ones. I suppose Xasmus for chiasmus might be a possibility. I've used XL8 for a jocular translate.

Ceci n'est pas un seing.