The Collaborative International Dictionary of English v.0.48:

Tarnish \Tar"nish\, v. i.
   To lose luster; to become dull; as, gilding will tarnish in a
   foul air.
   [1913 Webster]

         Till thy fresh glories, which now shine so bright,
         Grow stale and tarnish with our daily sight. --Dryden.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Tarnish \Tar"nish\, n.
   1. The quality or state of being tarnished; stain; soil;
      blemish.
      [1913 Webster]

   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.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

   Syn: To sully; stain; dim.
        [1913 Webster]

WordNet (r) 3.0 (2006):

tarnish
    n 1: discoloration of metal surface caused by oxidation
    v 1: 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: tarnish, stain,
         maculate, sully, defile]

Moby Thesaurus II by Grady Ward, 1.0:

143 Moby Thesaurus words for "tarnish":
   achromatize, asperse, aspersion, attaint, badge of infamy,
   bar sinister, baton, bedaub, befoul, begrime, benasty,
   bend sinister, besmear, besmirch, besmoke, besoil, bespatter,
   bestain, black eye, black mark, blacken, blanch, bleach, blemish,
   bloodstain, blot, blotch, blow upon, blur, brand, broad arrow,
   call names, calumniate, censure, champain, contaminate, corrupt,
   dab, damage, darken, daub, debase, decolor, decolorize, defame,
   defile, degrade, denigrate, dim, dirty, disapprove, discolor,
   disgrace, dishonor, disparage, disparagement, drain,
   drain of color, dull, embarrass, engage in personalities, etiolate,
   expose, expose to infamy, eyesore, fade, fleck, flick, flyspeck,
   foul, fume, gibbet, grime, hang in effigy, harm, heap dirt upon,
   hurt, impair, imputation, infect, injure, macula, maculation,
   macule, mar, mark, mark of Cain, mess, mess up, muckrake, muddy,
   nasty, onus, pale, patch, peroxide, pillory, pillorying,
   point champain, pollute, prejudice, reflection, reprimand,
   reproach, revile, ruin, scorch, sear, singe, slander, slubber,
   slur, smear, smirch, smoke, smooch, smouch, smudge, smut, smutch,
   soil, spatter, speck, speckle, splash, splatter, splotch, spoil,
   spot, stain, stigma, stigmatism, stigmatization, stigmatize, sully,
   taint, tar, throw mud at, tone down, vilify, vitiate, wash out,
   whiten