John Gill 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)

And they will deceive everyone his neighbour In conversation, with lying words; and in trade and commerce, by art and tricking.

and will not speak the truth with respect to facts they report, or goods they sell.

they have taught their tongue to speak lies and become so accustomed to lying that they cannot do otherwise; it is as it were natural to them.

and weary themselves to commit iniquity spared no pains to come at it, nor any in it, and go on even to weariness; are more laborious and indefatigable in committing sin than good men are in doing good; which shows great folly and stupidity. The Targum is, "they are become foolish, they have erred."