Other forms: gnashing
To gnash is to grind your teeth together in anger or distress. A sound as much as an action.
The word appears almost entirely in one phrase, since gnashing of teeth is a biblical expression for extreme misery and has stayed in English ever since. Modern use is usually half-joking, and there was much wailing and gnashing of teeth means people complained a great deal. The g is silent. Grinding your teeth in your sleep is the medical version, and it has its own name, bruxism.
1verbgnashTo grind the teeth together, especially in rage or anguish.
“He gnashed his teeth at every mention of the name.”
Middle English
gnasten
English, today
gnash
From Middle English gnasten. unknown; the word is probably either Germanic or ultimately imitative of the sound. Compare Old Norse gnastan, Danish gnaske ("munch", "crunch"), German knirschen, German Low German gnirschen (“gnash”), gnissla (“squeak; gnash”), all of onomatopoeic origin.
Etymology from Wiktionary, CC BY-SA