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 wheresoever it seizes upon him, it tears him, and he foams, and gnashes his teeth, and pines away. And I spake to Thy disciples, that they should cast it out; and they were not able."
Now, while hearing these things, they were being cut to their hearts, and were gnashing their teeth over him.