If you describe someone as unimpeachable, you mean that they are completely honest and reliable.
He said all five were men of unimpeachable character., ...an unimpeachable source.
Translation English Cobuild Collins Dictionary
above reproach, beyond criticism, beyond question, blameless, faultless, impeccable, irreproachable, perfect, unassailable, unblemished, unchallengeable, unexceptionable, unquestionable
blameworthy, faulty, imperfect, reprehensible, reproachable, shameful
English Collins Dictionary - English synonyms & Thesaurus
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