This part of the verse is identical with the LXX of Ps. 140:3. The poison of falsehood is as deadly as a serpent’s venom.
Have used deceit.
Literally, “were deceiving.” The tense denotes perseverance in the practice of deceit. The Hebrew of Ps. 5:9 means literally, “they made their tongues smooth,” that is, they used smooth, flattering words.
Open sepulchre.
As the open grave will soon be filled with death and corruption, so the throat of the wicked, opened for speech, is filled with corrupt and deadly falsehood. Compare Jer. 5:16, where the quiver of the Chaldeans is also called an open sepulcher. Some explain the figure to mean that their speech is like the odor of a newly opened tomb (see John 11:39).