to challenge as false (another's statements, motives, etc.); cast doubt upon.
2.
Archaic. to assail (a person) by words or arguments; vilify.
3.
Obsolete. to attack (a person) physically.
Origin: 1325–75; Middle English impugnen < Middle French impugner < Latin impugnāre to attack, equivalent to im-im-1 + pugnāre to fight, derivative of pugnus fist; see pugnacious