The Collaborative International Dictionary of English v.0.48:

Discredit \Dis*cred"it\, v. t. [imp. & p. p. Discredited; p.
   pr. & vb. n. Discrediting.] [Cf. F. discr['e]diter.]
   1. To refuse credence to; not to accept as true; to
      disbelieve; as, the report is discredited.
      [1913 Webster]

   2. To deprive of credibility; to destroy confidence or trust
      in; to cause disbelief in the accuracy or authority of.
      [1913 Webster]

            An occasion might be given to the . . . papists of
            discrediting our common English Bible. --Strype.
      [1913 Webster]

   2. To deprive of credit or good repute; to bring reproach
      upon; to make less reputable; to disgrace.
      [1913 Webster]

            He. . . least discredits his travels who returns the
            same man he went.                     --Sir H.
                                                  Wotton.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Discredit \Dis*cred"it\, n. [Cf. F. discr['e]dit.]
   1. The act of discrediting or disbelieving, or the state of
      being discredited or disbelieved; as, later accounts have
      brought the story into discredit.
      [1913 Webster]

   2. Hence, some degree of dishonor or disesteem; ill repute;
      reproach; -- applied to persons or things.
      [1913 Webster]

            It is the duty of every Christian to be concerned
            for the reputation or discredit his life may bring
            on his profession.                    --Rogers.

   Syn: Disesteem; disrepute; dishonor; disgrace; ignominy;
        scandal; disbelief; distrust.
        [1913 Webster]

WordNet (r) 3.0 (2006):

discredit
    n 1: the state of being held in low esteem; "your actions will
         bring discredit to your name"; "because of the scandal the
         school has fallen into disrepute" [syn: disrepute,
         discredit] [ant: reputation, repute]
    v 1: cause to be distrusted or disbelieved; "The paper
         discredited the politician with its nasty commentary"
    2: damage the reputation of; "This newspaper story discredits
       the politicians" [syn: discredit, disgrace]
    3: reject as false; refuse to accept [syn: disbelieve,
       discredit] [ant: believe]

Moby Thesaurus II by Grady Ward, 1.0:

156 Moby Thesaurus words for "discredit":
   agnosticism, asperse, aspersion, atheism, bad character, bad name,
   bad odor, bad report, bad reputation, bad repute, belie, belittle,
   besmirch, blacken, blemish, blot, blow sky-high, blow up, brand,
   bring down, bring into discredit, bring low, bring shame upon,
   bring to naught, calumniate, cast reproach upon, cry down, damage,
   debase, decry, defamation, defame, deflate, defrock, degradation,
   degrade, demean, denial, deny, deplume, deprecate, depreciate,
   derogate from, destroy, detract, detract from, devaluate, devalue,
   diminish, disapprobation, disapprove of, disbelief, disbelieve,
   disconfirm, disesteem, disfavor, disgrace, dishonor, disparage,
   displume, disprove, dispute, disrepute, distrust, doubt,
   doubtfulness, dubiousness, evil repute, explode, expose, gag on,
   harm, heresy, hold in contempt, humiliate, humiliation, ignominy,
   ill fame, ill repute, ill-favor, impute shame to,
   inability to believe, incredulity, infamy, infidelity, invalidate,
   knock, libel, lower, make little of, malign, minimifidianism,
   minimize, misbelief, misbelieve, mistrust, mock, negate, negative,
   nonbelief, not admit, not believe, not buy, not swallow,
   nullifidianism, obloquy, odium, opprobrium, pillory,
   poor reputation, prove the contrary, public dishonor, puncture,
   put down, put to shame, qualm, question, reduce,
   reflect discredit upon, reflection, refuse to admit,
   refuse to credit, refute, reject, rejection, reproach, ridicule,
   ruin, run down, scandal, scepticism, scruple, shady reputation,
   shame, shoot, show up, slander, slight, slur, smear, smirch,
   speak ill of, stigma, stigmatize, submit to indignity, sully,
   suspicion, taint, tarnish, unbelief, unbelieve, unbelievingness,
   undercut, unfrock, unsavory reputation, vilify