impugn
Concise Oxford English Dictionary © 2008 Oxford University Press:
impugn /ɪmˈpjuːn/
▶verb dispute the truth, validity, or honesty of (a statement or motive).
– derivatives
impugnable adjective,
impugnment noun.
impugnable adjective,
impugnment noun.
– origin ME: from L. impugnare ‘assail’.
'impugn' also found in these Oxford entries:

