Heb harak, meaning "to grate the teeth", (Job 16:9; Ps 112:10; La 2:16), denotes rage or sorrow. (See also Ac 7:54; Mr 9:18.)
And wherever it seizeth hold of him, it throweth him down; and he foameth and gnasheth his teeth; and he pineth away. And I spoke to thy disciples to cast it out, and they were not able.
But when they heard these things, their hearts were filled with rage, and they gnashed their teeth at him.