Psalms 140:3

They have sharpened their tongues like a serpent. The rapid motion of a viper's tongue gives you the idea of its sharpening it; even thus do the malicious move their tongues at such a rate that one might suppose them to be in the very act of wearing them to a point, or rubbing them to a keen edge. It was a common notion that serpents inserted their poison by their tongues, and the poets used the idea as a poetical expression, although it is certain that the serpent wounds by his fangs and not by his tongue.

They have sharpened their tongues like a serpent. The rapid motion of a viper's tongue gives you the idea of its sharpening it; even thus do the malicious move their tongues at such a rate that one might suppose them to be in the very act of wearing them to a point, or rubbing them to a keen edge. It was a common notion that serpents inserted their poison by their tongues, and the poets used the idea as a poetical expression, although it is certain that the serpent wounds by his fangs and not by his tongue. We are not to suppose that all authors who used such language were mistaken in their natural history any more than a writer can be charged with ignorance of astronomy because he speaks of the sun's travelling from east to west. How else can poets speak but according to the appearance of things to an imaginative eye. The world's great poet puts it in "King Lear": In the case of slanderers, they so literally sting with their tongues, which are so nimble in malice, and withal so piercing and cutting, that it is by no means unjust to speak of them as sharpened. "Adders' poison is under their lips." The deadliest of all venom is the slander of the unscrupulous. Some men care not what they say so long as they can vex and injure. Our text, however, must not be confined in its reference to some few individuals, for in the inspired epistle to the Romans it is quoted by the apostle as being true of us all. So depraved are we by nature that the most venomous creatures are our fit types. The old serpent has not only inoculated us with his venom, but he has caused us to be ourselves producers of the like poison: it lies under our lips, ready for use, and, alas, it is all too freely used when we grow angry, and desire to take vengeance upon any who have caused us vexation.

They have sharpened their tongues - They employ their time in forging lies and calumnies against me; and those of the most virulent nature.

They have sharpened their tongues like a serpent - Compare the notes at Psa 64:3. The idea here is, that since the tongue of the serpent “seems” to be sharp, pointed, adapted to penetrate (and probably the original reference in the image was derived from that idea), the wound inflicted is by the serpent’s tongue - “as if” with a hard, penetrating point. It is now known, however, that it is by a tooth - a single tooth, made flexible for the purpose - at the root of which a small bag containing the poison is located, which is injected through an orifice in the tooth into the wound. The meaning here is, that the words spoken by such persons - by their tongues - were like the poison produced by the bite of a serpent. Adders’ poison is under their lips - The asp or adder is among the most poisonous of serpents. Thus, Cleopatra of Egypt is said to have destroyed her own life by an asp, which she had concealed for that purpose. This passage is quoted in Rom 3:13, as a proof of human depravity. See the notes at that verse.