What is the X in X-mas?
"mark made on the skin by a whip," 1808, perhaps an alteration of wale, possibly by confusion with weal "welt," and obsolete wheal "pimple, pustule" (mid-15c.), from Old English verb hwelian "to form pus, bring to a head."
A small swelling on the skin, as from an insect bite, that usually itches or burns. Also called welt.