Thy tongue deviseth mischiefs; or, malignities—evils of the worst kind. It was Doeg's "tongue" that brought about the entire ghastly massacre (see 1 Samuel 22:9, 1 Samuel 22:10). Like a sharp razor, working deceitfully.
Doeg had "worked deceitfully," since he had not told Saul the circumstances that made Ahimelech's giving aid to David no disloyalty to the king (1 Samuel 21:2, 1 Samuel 21:8). The suppressio veri is a suggestio falsi.