Source : Webster's Revised Unabridged Dictionary (1913)

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

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

            To denigrate the memory of Voltaire.  --Morley.

Source : WordNet®

denigrate
     v 1: belittle; "Don't belittle his influence" [syn: {minimize}, {belittle},
           {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},
         {calumniate}, {smear}, {sully}, {besmirch}]