What is the X in X-mas?
late 14c., from Old French exceder (14c.) "exceed, surpass, go too far," from Latin excedere "depart, go beyond, be in excess, surpass," from ex- "out" (see ex-) + cedere "go, yield" (see cede).
Related: Exceeded; exceeding. Exceedingly (late 15c.) means "very greatly or very much;" excessively (mid-15c.) means "too greatly or too much."