What is the X in X-mas?
Old English grislic "horrible, dreadful," from root of grisan "to shudder, fear" (cf. Old Frisian grislik "horrible," Middle Dutch grisen "to shudder," Dutch griezelen, German grausen "to shudder, fear," Old High German grisenlik "horrible"), of unknown origin; Watkins connects it with the PIE root *ghrei- "to rub," on notion of "to grate on the mind." Cf. also gruesome, to which it probably is connected in some way.