The Collaborative International Dictionary of English v.0.48:

Impugn \Im*pugn"\, v. t. [imp. & p. p. Impugned; p. pr. & vb.
   n. Impugning.] [OE. impugnen, F. impugner, fr. L.
   impugnare; in on, against + pugnare to flight. See
   Pugnacious.]
   To attack by words or arguments; to contradict; to assail; to
   call in question; to make insinuations against; to gainsay;
   to oppose; as, to impugn a person's integrity.
   [1913 Webster]

         The truth hereof I will not rashly impugn, or
         overboldly affirm.                       --Peacham.
   [1913 Webster]

WordNet (r) 3.0 (2006):

impugn
    v 1: attack as false or wrong

Moby Thesaurus II by Grady Ward, 1.0:

73 Moby Thesaurus words for "impugn":
   abjure, accuse, anathematize, anathemize, animadvert on, arraign,
   assail, assert the contrary, attack, belie, blame,
   call in question, call to account, cast blame upon,
   cast reflection upon, censure, complain against, condemn, contest,
   contradict, contravene, controvert, counter, criminate, cross,
   cry down, cry out against, cry out on, cry shame upon, damn, decry,
   denounce, denunciate, deny, disaffirm, disallow, disavow, disclaim,
   disown, disprove, dispute, forswear, fulminate against, gainsay,
   impeach, implicate, incriminate, inculpate, indict,
   inveigh against, involve, join issue upon, negate, negative,
   not accept, not admit, nullify, oppose, recant, reflect upon,
   refuse to admit, refute, renounce, reprehend, reproach, reprobate,
   repudiate, retract, revoke, shake up, take back, take issue with,
   traverse