From Longman Dictionary of Contemporary Englishgnashgnash /næʃ/ verb → gnash your teeth→ See Verb table
Examples from the Corpus
gnash• The change was noted immediately, and my heavens, what a great gnashing of teeth and pulling of hair.• This sudden swing, forceful as a storm: the boy, gnashing teeth, snapping as an animal.• Their heads thrash about on the bloodied floor, gnashing their teeth and foaming at the mouth.Origin gnash (1400-1500) Probably from the sound 