clincher

This was a clincher—assorted worthies sent their own medals back in protest but everyone else was delighted.

The real clincher, though, is that their only instruments are a drum set and dozens of beer bottles.

The doubling of images of, say, the finger when the eyeball is pressed, is frequently proffered as a clincher.

In the South an expression of this kind is very often added on as a sort of clincher to give emphasis.