Charles Ellicott Commentary Psalms 5:9

"For there is no faithfulness in their mouth; Their inward part is very wickedness; Their throat is an open sepulchre; They flatter with their tongue." — Psalms 5:9 (ASV)

In their mouth. —See margin.

Wickedness. —Properly, an abyss, from the root “to fall,” therefore in parallelism with “open sepulcher” in the next clause. This is an instance of introverted parallelism, “mouth” answering to “tongue.” (See Bible Educator, iii. 50.)

An open sepulchre. —At once dangerous and noisome.

Flatter. —Literally, make smooth the tongue. (Compare Psalm 12:2.) Shakespeare uses “smooth tongue.” Compare also—

“The subtle fiend,
Though only strong with anger and disdain,
Dissembled, and this answer smooth returned.”

MILTON, Paradise Lost.