John Calvin Commentary Jeremiah 48:39

"How is it broken down! [how] do they wail! how hath Moab turned the back with shame! so shall Moab become a derision and a terror to all that are round about him." — Jeremiah 48:39 (ASV)

The Prophet still speaks in the person of others, and according to their feelings, not his own. He then says that, howling, they would exclaim in wonder, How is it that Moab has been so broken, that all had turned their backs, that Moab had become ashamed?

He indirectly implies that although no one could then know God's judgment, which he now foretells, God would nevertheless prove by the event that He had been entirely serious. This wonder, then, was expressed for this purpose: that the Jews might know that even though the calamity of Moab would fill everyone with astonishment, making them cry out regarding such an extraordinary thing, “What can this mean?” the fulfillment of his prophecy would still be certain.

This is the meaning of the words when he says, Howling, they will cry out, How has Moab been broken? and how has he turned his neck, or as they say, his back? Moab is ashamed; and then, he is made a derision, which we have observed before.

He adds, a terror, though some read “a bruising”; but fear or terror is more suitable. For the Prophet means that Moab would be a derision to others and a dread to others, being an example of God’s awful judgment. And he says that Moab would be a terror to all around—that is, to the whole surrounding country—as well as a subject of laughter and derision.