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 when he may take him, he lacerates him: and he froths, and gnashes his teeth, and pines away. And I said to thy disciples, that they should cast him out; and they were not able.
And hearing these words, they were cut through their hearts and gnashed their teeth at him.