If something is incontestable, it is so clearly true that you cannot honestly say it is wrong. The facts sit there; arguing against them looks foolish. You will hear it in law, in the news, and in careful debate: incontestable evidence, an incontestable fact. It is a strong, formal word — stronger than clear, close to you cannot deny it. In everyday speech people often say you can’t argue with that. Though actually, people do still argue; the word is a claim that the argument should already be over.