Source : Webster's Revised Unabridged Dictionary (1913)

Tarnish \Tar"nish\, v. i.
   To lose luster; to become dull; as, gilding will tarnish in a
   foul air.

         Till thy fresh glories, which now shine so bright, Grow
         stale and tarnish with our daily sight.  --Dryden.

Tarnish \Tar"nish\, n.
   1. The quality or state of being tarnished; stain; soil;
      blemish.

   2. (Min.) A thin film on the surface of a metal, usually due
      to a slight alteration of the original color; as, the
      steel tarnish in columbite.

Tarnish \Tar"nish\, v. t. [imp. & p. p. {Tarnished}; p. pr. &
   vb. n. {Tarnishing}.] [F. ternir, fr. OHG. tarnen to darken,
   to conceal, hide; akin to OS. dernian to hide, AS. dernan,
   dyrnan, OHG. tarni hidden, OS. derni, AS. derne, dyrne. Cf.
   {Dern}, a., and see {-ish}.]
   To soil, or change the appearance of, especially by an
   alternation induced by the air, or by dust, or the like; to
   diminish, dull, or destroy the luster of; to sully; as, to
   tarnish a metal; to tarnish gilding; to tarnish the purity of
   color. ``Tarnished lace.'' --Fuller. Used also figuratively;
   as, to tarnish one's honor.

   Syn: To sully; stain; dim.

Source : WordNet®

tarnish
     n : discoloration of metal surface caused by oxidation
     v : make dirty or spotty, as by exposure to air; also used
         metaphorically; "The silver was tarnished by the long
         exposure to the air"; "Her reputation was sullied after
         the affair with a married man" [syn: {stain}, {maculate},
          {sully}, {defile}]