The Collaborative International Dictionary of English v.0.48:

Besmirch \Be*smirch"\, v. t. [imp. & p. p. Besmirched; p. pr.
   & vb. n. Besmirching.]
   To smirch or soil; to discolor; to obscure. Hence: To
   dishonor; to sully. --Shak.
   [1913 Webster]

WordNet (r) 3.0 (2006):

besmirch
    v 1: charge falsely or with malicious intent; attack the good
         name and reputation of someone; "The journalists have
         defamed me!" "The article in the paper sullied my
         reputation" [syn: defame, slander, smirch, asperse,
         denigrate, calumniate, smear, sully, besmirch]
    2: smear so as to make dirty or stained [syn: smirch,
       besmirch]

Moby Thesaurus II by Grady Ward, 1.0:

62 Moby Thesaurus words for "besmirch":
   attaint, bedarken, bedaub, besmear, besmoke, besmutch, besoil,
   bespatter, bestain, black, blacken, blackwash, blot, blotch, blur,
   brand, call names, charcoal, cork, darken, daub, defile, denigrate,
   dinge, dirty, discolor, ebonize, engage in personalities,
   heap dirt upon, ink, mark, melanize, muckrake, murk, nigrify,
   oversmoke, revile, scorch, sear, shade, shadow, singe, slubber,
   slur, smear, smirch, smoke, smouch, smudge, smut, smutch, soil,
   soot, spot, stain, stigmatize, sully, taint, tar, tarnish,
   throw mud at, vilify