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"