Jeremiah 3:23 - Ellicott's Commentary On The Whole Bible

Truly in vain... — The italics show the difficulty of the verse, and represent an attempt to get over it. According to the senses given to the word translated “multitude” we get, in vain (literally, as a lie) from the hills is the revelry (as in Amos 5:23), or the wealth, or the multitude, of the mountains. The first gives the best meaning, and expresses the confession of the repentant Israelites that their wild ritual on the high places had brought them loss and not gain.