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.)
"His anger has torn me and hunted me down, He has gnashed at me with His teeth; My adversary glares at me.
The wicked will see it and be vexed, He will gnash his teeth and melt away; The desire of the wicked will perish.
All your enemies Have opened their mouths wide against you; They hiss and gnash their teeth They say, "We have swallowed her up! Surely this is the day for which we waited; We have reached it, we have seen it."
and whenever it seizes him, it slams him to the ground and he foams at the mouth, and grinds his teeth and stiffens out. I told Your disciples to cast it out, and they could not do it."
Now when they heard this, they were cut to the quick, and they began gnashing their teeth at him.