Clincher

that which rivets or confirms an argument, an incontrovertible position. Also a lie which cannot be surpassed, a stopper-up, said to be derived as follows:—Two notorious liars were backed to outlie each other. “I drove a nail through the moon once,” said the first. “Right,” said the other; “I recollect the circumstance well, for I went round to the back part of the moon and clinched  it”—hence clincher.