We don't have synonyms for impugns yet — it may be a rare, specialized, newly coined, or misspelled word. The closest thesaurus entries we have are impugn.
(v) :
(transitive) To verbally assault, especially to argue against an opinion, motive, or action; to question the truth or validity of; cast doubt on.
(v) :
(transitive) To verbally assault, especially to argue against an opinion, motive, or action; to question the truth or validity of; cast doubt on.