Carl Friedrich Keil and Franz Delitzsch Old Testament Commentary

The verb זמם is construed with ל of that which is the object at which the evil devices aim. To gnash the teeth (elsewhere also: with the teeth) is, as in Psa 35:16, cf. Job 16:9, a gesture of anger, not of mockery, although anger and mockery are usually found together. But the Lord, who regards an assault upon the righteous as an assault upon Himself, laughs (Psa 2:4) at the enraged schemer; for He, who orders the destinies of men, sees beforehand, with His omniscient insight into the future, his day, i.e., the day of his death (Sa1 26:10), of his visitation (Psa 137:7, Oba 1:12, Jer 50:27, Jer 50:31).