Charles H. Spurgeon on 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. 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":

"She struck me with her tongue,

Most serpent like, upon the very heart."

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. It is sadly wonderful what hard things even good men will say when provoked; yea, even such as call themselves "perfect" in cool blood are not quite as gentle as doves when their claims to sinlessness are bluntly questioned. This poison of evil speaking would never fall from our lips, however much we might be provoked, if it were not there at other times; but by nature we have as great a store of venomous words as a cobra has of poison. O Lord, take the poison bags away, and cause our lips to drop nothing but honey. Selah. This is heavy work. Go up, go up, my heart! Sink not too low. Fall not into the lowest key. Lift up thyself to God.