King James Bible Dictionary

Heb. harak, meaning "to grate the teeth", (Job 16:9; Psalms 112:10; Lamentations 2:16), denotes rage or sorrow. (See also Acts 7:54; Mark 9:18.)

GNASH, verb transitive nash. To strike the teeth together, as in anger or pain; as, to gnash the teeth in rage.

GNASH, verb intransitive nash. To grind the teeth.

He shall gnash with his teeth and melt away. Psalms 112:10.

1. To rage even to collision with the teeth; to growl.

They gnashed on me with their teeth. Psalms 35:16.

GNASH'ING, participle present tense nash'ing. Striking the teeth together, as in anger, rage or pain.

GNASH'ING, noun nash'ing. A grinding or striking of the teeth in rage or anguish.

There shall be weeping and gnashing of teeth. Matthew 8:12.

General references
Job 16:9; Psalms 35:16; Psalms 37:12; Psalms 112:10; Lamentations 2:16; Matthew 8:12; Matthew 13:42; Matthew 22:13; Matthew 24:51; Matthew 25:30; Luke 13:28
Teeth