mid-15c. ternishen, "become tarnished; discolor," from Old French terniss-, present-participle stem of ternir "dull the luster or brightness of, make dim" (15c.), probably from terne (adj.) "dull, dark," which according to Diez is 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), but there are difficulties of form, sense, and date. Figurative sense is from 1690s. Related: Tarnished; tarnishing.