Source : Webster's Revised Unabridged Dictionary (1913)

Gnash \Gnash\, v. t. [imp. & p. p. {Gnashed}; p. pr. & vb. n.
   {Gnashing}.] [OE. gnasten, gnaisten, cf. Icel. gnastan a
   gnashing, gn?sta to gnash, Dan. knaske, Sw. gnissla, D.
   knarsen, G. knirschen.]
   To strike together, as in anger or pain; as, to gnash the
   teeth.

Gnash \Gnash\, v. i.
   To grind or strike the teeth together.

         There they him laid, Gnashing for anguish, and despite,
         and shame.                               --Milton.

Source : WordNet®

gnash
     v : grind together, of teeth