gnash 
(năsh)
(năsh)tr.v. gnashed, gnash·ing, gnash·es
1. To grind or strike (the teeth, for example) together.
2. To bite (something) by grinding the teeth.
[Alteration of Middle English gnasten, gnaisten, possibly of Scandinavian origin; akin to Old Norse gnastan, a gnashing.]
gnash n.