In a way that leaves no room for criticism or doubt. It is rare, and it almost always comes before an adjective in careful writing: unimpeachably honest, unimpeachably fair, unimpeachably correct, unimpeachably sourced. The effect is stronger than "very" — the writer is saying that even a hostile person, looking for a fault, would find none. Because it is long and formal, using it in ordinary conversation sounds bookish, and students should expect to read it far more often than they say it.

無可指摘地；無懈可擊地。中文通常譯成「無可挑剔地誠實／公正」，或乾脆改成「誠實得無可挑剔」。