Albert Barnes Commentary Psalms 37:12

"The wicked plotteth against the just, And gnasheth upon him with his teeth." — Psalms 37:12 (ASV)

The wicked plotteth against the just - Margin, “practices.” The Hebrew word means to plot, to lie in wait, to plan, to purpose, and to devise. See (Psalms 31:13). The meaning is that wicked people lay their plans against the righteous, but they will not be able to carry them out or accomplish them, for they will be cut off, and the Lord will protect His friends.

And gnasheth upon him with his teeth - An expression of rage or anger. See the notes at (Psalms 35:16).