John Gill Commentary Psalms 37:12

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

The wicked plots against the just
Forms schemes to make him uneasy, uncomfortable, and unhappy:

and gnashes upon him with his teeth :
which expresses his malice and hatred, as David's enemies did upon him, (Psalms 35:16) ; and Stephen's, (Acts 7:54) .