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 seizes him it dashes him down; he foams at the mouth, and grinds his teeth, and is wasting away. I begged your disciples to cast it out, but they could not."
As they heard these words they became furious and gnashed their teeth at him.