The Collaborative International Dictionary of English v.0.48:

Denigrate \Den"i*grate\, v. t. [L. denigrare; de- + nigrare to
   blacken, niger black.]
   1. To blacken thoroughly; to make very black. --Boyle.
      [1913 Webster]

   2. Fig.: To blacken or sully; to defame. [R.]
      [1913 Webster]

            To denigrate the memory of Voltaire.  --Morley.
      [1913 Webster]

WordNet (r) 3.0 (2006):

denigrate
    v 1: cause to seem less serious; play down; "Don't belittle his
         influence" [syn: minimize, belittle, denigrate,
         derogate]
    2: 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]