John Calvin Commentary Jeremiah 9:8

"Their tongue is a deadly arrow; it speaketh deceit: one speaketh peaceably to his neighbor with his mouth, but in his heart he layeth wait for him." — Jeremiah 9:8 (ASV)

The Prophet again complains of the deceitfulness of their tongues; and he compares them to deadly or drawn-out arrows. Gold is said to be drawn out when refined by repeated meltings; similarly, arrows, when sharpened, are more piercing. The Prophet then says that their tongues were like deadly or sharpened arrows. How so? Because they always spoke deceit, by either slandering or circumventing others. But the expression is general, and the Prophet no doubt meant to include all ways of deceiving.

For it then follows, With the mouth they speak peace; that is, everyone professed friendship, and his words were honey; and yet within he did set up, or concealed intrigues. Here, in other words, he describes their treachery, because the tongue and the heart differed. They showed by the tongue what was different from the true feelings of their heart. Therefore, he says that they set up treacheries within them, or in their hearts, while they spoke peace with the mouth—that is, pretended brotherly kindness. Finally, he repeats what he had said before (Jeremiah 5:9).