E. Cobham Brewer 1810–1897. 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.)
1
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.)
2
I gave him a clencher (should be “clincher”). I nailed him fast.
