Tarnish
late Middle English (as a verb): from French terniss-, lengthened stem of ternir, from terne ‘dark, dull’.
wiktionary
From Middle English ternysshen, a borrowing from Old French terniss-, stem of ternir(“to make dim, make wan”), borrowed from Frankish *darnijan(“to conceal”). Doublet of dern and darn.
etymonline
tarnish (v.)
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.
tarnish (n.)
1713, from tarnish (v.).
