Thy tongue deviseth mischiefs - The word rendered “mischiefs” means
(a) desire, cupidity: Proverbs 10:3; then
(b) fall, ruin, destruction, wickedness: Psalms 5:9; Psalms 38:12.
The meaning here is, that he made use of his tongue to ruin others. Compare Psalms 50:19. The particular thing referred to here is the fact that Doeg sought the ruin of others by giving “information” in regard to them. He “informed” Saul of what Ahimelech had done; he informed him where David had been, thus giving him, also, information in what way he might be found and apprehended. All this was “designed” to bring ruin upon David and his followers. It “actually” brought ruin on Ahimelech and those associated with him, 1 Samuel 22:17-19.
Like a sharp razor - See the notes at Isaiah 7:20. His slanders were like a sharp knife with which one stabs another. So we stay of a slanderer that he “stabs” another in the dark.
Working deceitfully - literally, making deceit. That is, it was by deceit that he accomplished his purpose. There was no open and fair dealing in what he did.