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]

Moby Thesaurus II by Grady Ward, 1.0:

40 Moby Thesaurus words for "impugned":
   accused, arraigned, belied, blamed, charged, cited, confounded,
   confuted, deflated, denied, denounced, discarded, discredited,
   dismissed, disproved, disputed, exploded, exposed, impeached,
   implicated, in complicity, incriminated, inculpated, indicted,
   invalidated, involved, negated, negatived, overthrown, overturned,
   punctured, refuted, rejected, reproached, shown up, tasked, taxed,
   under attack, under fire, upset