John Calvin Commentary Jeremiah 5:8

"They were as fed horses roaming at large; every one neighed after his neighbor`s wife." — Jeremiah 5:8 (ASV)

Jeremiah comes now, I think, to the second table, and mentions one kind of evil; but his object was to show that there was no chastity, no faithfulness, no honesty among that people. He therefore compares them to wanton and lustful horses, and thus exposes their infamous conduct.

Had he said that everyone lay in wait for his neighbor's bed, it would have been a heinous crime. But when he calls their lust neighing, and calls them horses, and those well fed, and who rise early after they are filled, he doubtless shows that their incontinence was such that they were not only wanton and adulterous, but they were worse and more base, for they differed in nothing from lustful horses, and horses well fed.

Some read the last word “armed,” deducing it from יון izan, which means to be armed; and others derive it from זון, zun, which signifies to eat, and hence they take מוזן, muzan, for food. There is indeed no doubt that it means here “fed” or “fat”; for why should he call them armed horses? What some say—that they rose early after having committed adultery, in order to exhibit their disgrace and to boast of their vices—is too far-fetched. What is meant is that they were strong and active horses, and that they rose up early after having been well fed.

We now, then, understand the Prophet’s object: the sum of the whole is that there was no chastity among the Jews, for they gave themselves up to wanton lusts, not only like adulterers and whoremongers, but like lascivious horses. Nevertheless, as we have said, he includes here, under incontinence, thefts, frauds, rapines, and all vices of this kind; for he undoubtedly charges the Jews with transgressing against the second table of the law. He afterwards adds—