impugn
verb /ɪmˈpjuːn/
  /ɪmˈpjuːn/
(formal)Verb Forms
| present simple I / you / we / they impugn |    /ɪmˈpjuːn/   /ɪmˈpjuːn/  | 
| he / she / it impugns |    /ɪmˈpjuːnz/   /ɪmˈpjuːnz/  | 
| past simple impugned |    /ɪmˈpjuːnd/   /ɪmˈpjuːnd/  | 
| past participle impugned |    /ɪmˈpjuːnd/   /ɪmˈpjuːnd/  | 
| -ing form impugning |    /ɪmˈpjuːnɪŋ/   /ɪmˈpjuːnɪŋ/  | 
- impugn something to express doubts about whether something is right, honest, etc. synonym challenge
- There were no real grounds for impugning the decision.
 
Oxford Collocations DictionaryImpugn is used with these nouns as the object:- integrity
 - motive
 
Word Originlate Middle English (also in the sense ‘assault, attack physically’): from Latin impugnare ‘assail’, from in- ‘towards’ + pugnare ‘fight’.