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.)
Wherever it seizes him, it convulses him, and he foams at the mouth and grinds his teeth; and he is wasting away. So I asked your disciples to drive it out, but they could not do it."
As they continued to listen to this address, they were becoming infuriated and began to grind their teeth at him.