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.)
He hath torn me in his wrath, and persecuted me; He hath gnashed upon me with his teeth: Mine adversary sharpeneth his eyes upon me.
The wicked shall see it, and be grieved; He shall gnash with his teeth, and melt away: The desire of the wicked shall perish.
All thine enemies have opened their mouth wide against thee; They hiss and gnash the teeth; they say, We have swallowed her up; Certainly this is the day that we looked for; we have found, we have seen it.
and wheresoever it taketh him, it dasheth him down: and he foameth, and grindeth his teeth, and pineth away: and I spake to thy disciples that they should cast it out; and they were not able.
Now when they heard these things, they were cut to the heart, and they gnashed on him with their teeth.
American Standard Version Public Domain