What is the X in X-mas?
Old English hearm "hurt, evil, grief, pain, insult," from Proto-Germanic *harmaz (cf. Old Saxon harm, Old Norse harmr, Old Frisian herm "insult; pain," Old High German harm, German Harm "grief, sorrow, harm"), from PIE *kormo- "pain."
Old English hearmian "to hurt" (see harm (n.)). It has ousted Old English skeþþan "scathe" in all but a few senses. Related: Harmed; harming.