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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"Collins Cobuild English Dictionary for Advanced Learners 4th edition published in 2003 © HarperCollins Publishers 1987, 1995, 2001, 2003 and Collins A-Z Thesaurus 1st edition first published in 1995 © HarperCollins Publishers 1995"