E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Clench and Clinch.
To clench is to grasp firmly, as, He clenched my arm firmly, He clenched his nerves bravely to endure the pain. (Anglo-Saxon, be-clencan, to hold fast.)
To clinch is to make fast, to turn the point of a nail in order to make it fast. Hence, to clinch an argument. (Dutch, klinken, to rivet. Whence clinker-built, said of a ship whose planks overlap each other, and are riveted together.)
I gave him a clencher (should be clincher). I nailed him fast.