1. Impossible to be discredited or challenged: Clifford's information was completely true because it came from an unquestionable and unimpeachable source.
2. Not liable to accusation or denunciation; irreproachable: Jack Star had an unimpeachable reputation because he was reliable, dependable, honest, and trustworthy.
3. Entirely acceptable: When Albert finished his project it was done adequately, up to the teacher's expectations, and was regarded as being unimpeachable, with no parts missing or being imperfect.