Source : Webster's Revised Unabridged Dictionary (1913)

Dissimilate \Dis*sim"i*late\, v. t.
   To render dissimilar.

Source : WordNet®

dissimilate
     v 1: become dissimilar by changing the sound qualities; "These
          consonants dissimilate" [ant: {assimilate}]
     2: make dissimilar; cause to become less similar [ant: {assimilate}]
     3: become dissimilar or less similar; "These two related tribes
        of people gradually dissimilated over time" [ant: {assimilate}]