The Collaborative International Dictionary of English v.0.48:

Impeach \Im*peach"\, v. t. [imp. & p. p. Impeached; p. pr. &
   vb. n. Impeaching.] [OE. empeechier to prevent, hinder,
   bar, F. emp[^e]cher, L. impedicare to entangle; pref. im- in
   + pedica fetter, fr. pes, pedis, foot. See Foot, and
   Appeach, Dispatch, Impede.]
   1. To hinder; to impede; to prevent. [Obs.]
      [1913 Webster]

            These ungracious practices of his sons did impeach
            his journey to the Holy Land.         --Sir J.
                                                  Davies.
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            A defluxion on my throat impeached my utterance.
                                                  --Howell.
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   2. To charge with a crime or misdemeanor; to accuse;
      especially to charge (a public officer), before a
      competent tribunal, with misbehavior in office; to cite
      before a tribunal for judgment of official misconduct; to
      arraign; as, to impeach a judge. See Impeachment.
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   3. Hence, to charge with impropriety; to dishonor; to bring
      discredit on; to call in question; as, to impeach one's
      motives or conduct.
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            And doth impeach the freedom of the state. --Shak.
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   4. (Law) To challenge or discredit the credibility of, as of
      a witness, or the validity of, as of commercial paper.
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   Note: When used in law with reference to a witness, the term
         signifies, to discredit, to show or prove unreliable or
         unworthy of belief; when used in reference to the
         credit of witness, the term denotes, to impair, to
         lessen, to disparage, to destroy. The credit of a
         witness may be impeached by showing that he has made
         statements out of court contradictory to what he swears
         at the trial, or by showing that his reputation for
         veracity is bad, etc.

   Syn: To accuse; arraign; censure; criminate; indict; impair;
        disparage; discredit. See Accuse.
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The Collaborative International Dictionary of English v.0.48:

Impeach \Im*peach"\, n.
   Hindrance; impeachment. [Obs.]
   [1913 Webster]

WordNet (r) 3.0 (2006):

impeach
    v 1: challenge the honesty or veracity of; "the lawyers tried to
         impeach the credibility of the witnesses"
    2: charge (a public official) with an offense or misdemeanor
       committed while in office; "The President was impeached"
    3: bring an accusation against; level a charge against; "The
       neighbors accused the man of spousal abuse" [syn: accuse,
       impeach, incriminate, criminate]

Moby Thesaurus II by Grady Ward, 1.0:

80 Moby Thesaurus words for "impeach":
   accuse, allege, anathematize, anathemize, animadvert on, arraign,
   article, asperse, attack, belittle, blame, book, bring accusation,
   bring charges, bring to book, call into question, call to account,
   cast aspersions on, cast blame upon, cast reflection upon, censure,
   challenge, charge, cite, complain, complain against, condemn,
   criminate, cry down, cry out against, cry out on, cry shame upon,
   damn, declaim, decry, denounce, denunciate, deprecate, discredit,
   disparage, fasten on, fasten upon, file a claim, finger,
   fulminate against, hang something on, have up, implicate, imply,
   impugn, impute, incriminate, inculpate, indict, inform against,
   inform on, insinuate, inveigh against, lay charges,
   lodge a complaint, lodge a plaint, malign, pin on, prefer charges,
   press charges, pull up, put on report, reflect upon, report,
   reprehend, reproach, reprobate, shake up, slander, take to task,
   task, taunt with, tax, twit, vilify