The Collaborative International Dictionary of English v.0.48:

Disbelieve \Dis`be*lieve"\, v. t. [imp. & p. p. Disbelieved;
   p. pr. & vb. n. Disbelieving.]
   Not to believe; to refuse belief or credence to; to hold not
   to be true or actual.
   [1913 Webster]

         Assertions for which there is abundant positive
         evidence are often disbelieved, on account of what is
         called their improbability or impossibility. --J. S.
                                                  Mill.
   [1913 Webster]

WordNet (r) 3.0 (2006):

disbelieve
    v 1: reject as false; refuse to accept [syn: disbelieve,
         discredit] [ant: believe]