Charles Ellicott Commentary Jeremiah 9:5

"And they will deceive every one his neighbor, and will not speak the truth: they have taught their tongue to speak lies; they weary themselves to commit iniquity." — Jeremiah 9:5 (ASV)

Deceive. —The word is commonly translated, as in the margin, mock. (So in 1 Kings 18:27; Judges 16:10; Judges 16:13; Judges 16:15.) The context here shows, however, that the kind of mockery is that which at once deludes and derides; and as the former meaning is predominant, the text of the English version is better left as it is.

To commit iniquity. —Literally, to go crookedly, or, in the strict sense of the word, to do wrong.