What is the X in X-mas?
Old English milde "gentle, merciful," from Proto-Germanic *milthjaz- (cf. Old Norse mildr, Old Saxon mildi, Old Frisian milde, Middle Dutch milde, Dutch mild, Old High German milti, German milde "mild," Gothic mildiþa "kindness"), from PIE *meldh-, from root *mel- "soft," with derivatives referring to soft or softened materials (cf. Greek malthon "weakling," myle "mill;" Latin molere "to grind;" Old Irish meldach "tender;" Sanskrit mrdh "to neglect," also "to be moist"). Originally of persons and powers; of the weather from c.1400, of disease from 1744. Also in Old English as an adverb, "mercifully, graciously."