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 throws him into convulsions; and he foams, and gnashes with his teeth, and pines away: and I spoke to your disciples to cast him out, and they were not able.
"When they heard these things, they were enraged in their hearts, and gnashed upon him with their teeth.