Easton's Bible Dictionary (1897)
Heb. harak, meaning “to grate the teeth”, (Job 16:9; Ps. 112:10; Lam. 2:16), denotes rage or sorrow. (See also Acts 7:54; Mark 9:18.)
Schaff's Bible Dictionary
GNASH, GNASH'ING, a striking or grinding of the teeth in the paroxysms of anguish or despair. Ps 112:10;Matt 8:12.