Hehehe. Let the evil laughter commence, even before I start to discuss the word, because I love this word...
Impugn comes from the Latin "in" (against) and "pugnare" (to fight, from "pugnus" for fist), meaning literally, to fight against physically, so it should come as no surprise that it has come to mean now just "to attack by words or argument, to make insinuations as to credibility", having lost the physical attack, although one of the obsolete meanings was to physically fight against. As I continue to explore these wotd, I find with increasing frequency that etymologies which had an element of physicality against someone became solely a verbal issue. Something to consider.
Of course, one of my favorite usages is the "archaic" one, about "to vilify", and there is actually a statement on the record in a deposition where I told Plaintiff's counsel "not to impugn my good character" with his remarks about the nature of my questions to his client. It was an ad hominum attack because he wanted to try to throw me off my game since it wasn't going so well for his client. Ultimately, got the case dismissed on a motion for summary judgment, which has about a 1 in 20 chance of succeeding.
But in more regular usage, all cross-examination implicitly impugns the credibility of the evidence being presented, and perhaps impugns the credibility of the person testifying as well. There distinction between the "archaic" usage and the common definition is whether the comment is directly or indirectly attacking the person. "You're stupid" or "didn't they teach you to xxx in law school" directly impugns, while "how could you know the color of the car when you were distracted and looking in the other direction" indirectly impugns. Now, since impugn calls into question credibility, it can only be used with the intangibles of people (statements, ethics, motivations). The obsolete usage of to physically attack someone is still between people. The lion does not impugn the lion tamer, even in the obsolete. Nor does the boxer doesn't usually impugn his opponent, unless it is a grudge match, because it still implies an element that there is a personal matter. After impugning the character of the lady, her champion took up a sword to impugn the scoundrel. Yes, getting at both usages. And finally, I regularly impugn the propriety of using words incorrectly. Enjoy.