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