Psalms 5:9

For [there is] no faithfulness in their mouth
In the mouth of the ungodly, as the Chaldee paraphrase; in the mouth of every one of them, as Aben Ezra interprets it: that is, in the mouth of every wicked, bloody, and deceitful man; of everyone of David's enemies, as Saul, or the conspirators with Absalom his son. There was no steadfastness in them; nothing right, sure, or firm said by them; nothing that could be depended upon; there was "no truth" in them, as the Septuagint, Vulgate Latin, Arabic, and Ethiopic versions render it. And as this was true of David's, so of Christ's enemies; of Herod, and of the Herodians, and of the Scribes and Pharisees; see ( Matthew 2:8 ) ( 22:16 ) ; and of the enemies of Christ's Gospel; of all false teachers, who speak not the word faithfully; tell dreams, use the hidden things of dishonesty, walk in craftiness, handle the word of God deceitfully, and speak lies in hypocrisy: there is no certainty in them, nor is any thing they say to be trusted to. And this is the character of wicked men in common: they are an assembly of treacherous men; there is none upright among them; nor is any confidence to be placed in them: the people of God are in danger of being imposed upon and misled by them to their hurt; and therefore the psalmist prays, that the Lord would lead him in his righteousness;

their inward part [is] very wickedness;
their heart, which is desperately wicked, exceeding sinful; not only wicked, and very wicked, but wickedness itself. This is the root and fountain of all wickedness, and the reason why there is no faithfulness in their mouth: the word

their throat [is] an open sepulchre;
or as one, as the Targum paraphrases it; to which the throat of wicked men may be compared for its voracity and insatiableness; the grave being one of those three or four things, which never has enough or is satisfied. And this is true of the throat, whether it be considered as an instrument of speech, and throws out devouring words to the prejudice of the characters and reputations of others; or as an instrument of swallowing meat and drink, and where the pleasure of appetite is; and so may be expressive of the eager desire of the wicked after sin, who drink up iniquity like water; and of their delight in it, and their fulness of it, and yet still greedy, insatiable, and not to be satisfied: and their throat may be compared to an open grave for the nauseous stench it emits; corrupt communication, filthiness, and foolish talking, proceeding out of it; and horrible oaths, curses, imprecations, and blasphemies, being belched out through it; and for the danger which is by it, since into it men may fall unawares; and so the evil communications of wicked men corrupt good manners, and do great mischief to those who fall into company with them;

they flatter with their tongue;
or, "make it smooth" ^F15; use oily expressions, soft language: or, "part" or "divide" ^F16 their tongue; are double tongued and double hearted; and so deceive persons, as the apostle interprets it in ( Romans 3:13 ) . They flatter God himself, drawing nigh to him in an hypocritical way; they flatter men, their neighbours, and impose upon them; they flatter princes, and such parasitic people were about David. And such are false teachers, who prophesy smooth things, and with good words and fair speeches deceive the hearts of the simple; or, which account there is great reason to pray to be led and directed by the Lord.

F13 (Mbrq) (h kardia) , Sept. "cor", V. L. i.e. "cogitatio", Muis.
F14 (twwh) "aerumuae", Junius & Tremellius, Piscator, Gejerus; "calamitates", Cocceius; so Ainsworth.
F15 (Nwqylx) "levigant, seu emolliunt", Piscator, Gejerus.
F16 "Divident vel dispertient", Mariana.