Psalms 140:3

They have sharpened their tongues like a serpent; adders’ poison is under their lips. Selah.

Harmful speech is like the bite of a serpent, for perverse words from the tongue sting like an adder and wound and injure those who hear them. That injury is deceitful, since such words can sound smooth while concealing malice and being used to harm others.
The poison under the tongue pictures an unbelieving servant censuring his fellow servants, as unbelief takes hold of him and his conduct declines. The deadly poison beneath the lips also describes skeptical and unbelieving books, whose influence is poison that weakens religious faith, fills the mind with doubt and skepticism, and turns the affections away from God.