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 comes upon him, it dashes him to the ground, and he foams at the mouth and grinds his teeth, and he is pining away. I begged your disciples to expel it, but they had not the power."
As they listened to these words, they became infuriated and gnashed their teeth at him.