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 whenever it seizes him it throws him down, and he foams at the mouth and grinds his teeth. He is wasting away with it; so I told your disciples to cast it out, but they could not."
When they heard this, they were furious and gnashed their teeth at him.