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]