Definition from Wiktionary, the free dictionary
- Homophone: Nash
- (transitive) To grind (one's teeth) in pain or in anger.
- to gnash the teeth
- (transitive) To grind between the teeth.
- to gnash the air in fury
- to gnash a carpet
- (Geordie) To run away
Derived terms 
to grind one's teeth in pain or in anger