To impugn something, such as a person's integrity, judgment, etc., is to attack or oppose it as false or lacking integrity.

// The defense attorneys did their best to impugn the motives of the prosecution’s key witnesses.

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“[Singer, Grace] Cummings is not shy about courting legendary company. After all, the protagonist of ‘Ramona,’ a smoldering pseudo-goth number that ultimately flames into a full torch song, is borrowed from Bob Dylan.... There’s a bit of Johnny Cash’s ‘Cry, Cry, Cry,’ toward the end of ‘Everybody’s Somebody,’ which borrows the sound of Memphis’ Stax rather than its Sun to impugn a wayward partner.” — Grayson Haver Currin, Pitchfork, 12 Apr. 2024

Impugn, pugnacious, pugilist: them’s fightin’ words, literally. All three words trace back to the Latin noun pugnus, meaning “fist.” Though they floated like butterflies down different paths into English from that shared source, each stings like a bee (so to speak) in its own way. The noun pugilist refers to a fighter or boxer, while the adjective pugnacious describes someone showing a readiness or desire to fight or argue. The verb impugn today has belligerent implications, albeit of the verbal jousting kind, but in its earliest known English uses in the 1300s, impugn could refer to a physical attack (as in, “the troops impugned the city”) as well as to assaults involving verbal contradiction or dispute. Over time, though, the sense of literal battling has become obsolete while the “assailing by words or arguments” sense has endured.