unimpeachable

Not able to be impeached or reproached.

a gentleman of unimpeachable integrity

“Patriotism” makes for a slogan that is so unimpeachable it’s arguably even manipulative; as a matter of strategy, it’s harder for opponents to vote against the USA PATRIOT Act than, say, the Enhanced Security Act.

Blameless.

Beyond doubt.

an unimpeachable alibi

On the previous day, the prisoner had purchased strychnine at the village chemist’s shop, wearing a disguise by means of which he hoped to throw the onus of the crime upon another man—to wit, Mrs. Inglethorp’s husband, of whom he had been bitterly jealous. Luckily for Mr. Inglethorp, he had been able to produce an unimpeachable alibi.