What is the X in X-mas?
1590s, from present participle stem of Middle French ternir "dull the luster or brightness of, make dim" (15c.), probably from Old French terne (adj.) "dull, dark," from a Germanic source cognate with Old High German tarnjan "to conceal, hide," Old English dyrnan "to hide, darken," from Proto-Germanic *darnjaz (see dern). Figurative sense is from 1690s. Related: Tarnished; tarnishing.
1713, from tarnish (v.).