To impugn someone is to raise serious doubts about them — their honesty, their courage, their skill, or why they did a thing. You are not only disagreeing with an idea; you are picking at the person (or the claim) behind it. It is a formal, almost courtroom verb. Lawyers, politicians, and newspapers use it. Ordinary conversation almost never does; people say question, attack, or “are you saying I’m lying?” It often takes the thing being doubted: impugn someone’s honesty, impugn what a witness said. You can impugn a person, but English more often impugns a quality or a statement. Though actually, the word can sound heavier than the speaker means. To impugn is to cast doubt, not always to prove the person is bad. It is not a punch, and it is not a casual insult. Insult throws dirt; impugn tries to make other people stop trusting.