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. so good or definite that criticism or doubt is impossible
completely acceptable; not open to exception or reproach; "two unexceptionable witnesses"; "a judge's ethics should be unexceptionable"
Not impeachable; not to be called in question; exempt from liability to accusation; free from stain, guilt, or fault; irreproachable; blameless; as, an unimpeachable reputation; unimpeachable testimony