John Gill, on Lam 3:59

Render unto them a recompence, O Lord, according to the work of their hands. The Septuagint and Vulgate Latin versions render this, and the following verses, not as petitions, but as prophecies of what should be; but they seem rather to be expressed by way of request; and here, that God would deal with them according to the law of retaliation, and requite them according to what they had done; that he would do to them as they had done to the Lord's people, and others; and this is ordered to be done particularly to the Chaldeans, or Babylonians, Jer 50:15. . Lamentations 3:65 lam 3:65 lam 3:65 lam 3:65Give them sorrow of heart,.... That which will cause sorrow of heart; such judgments and punishments as will be grievous to them. Some have observed a likeness between the word here used and that translated "music", Lam 3:63; and think some respect may be had to it; that whereas the people of God had been matter of mirth and music to them, God would give them music, but of another sort; a song, but a doleful one. The Septuagint version renders it, "a covering of the heart"; the word (a) having the signification of a shield, which covers; and may signify blindness, hardness, and stupidity of heart, that they might not see the evils coming upon them, and how to escape them. A modern learned interpreter, Christianus Benedictus Michaelis, would have it compared with the Arabic word , "ganan", which signifies "to be mad", and from whence is "muganah", "madness"; and so the sense be, give them distraction of mind: lay curse unto them: and what greater curse is there than to be given up to judicial blindness and hardness of heart, or to madness and distraction? it may include all the curses of the law denounced against transgressors. (a) , "tegumentum cordis", Montanus, Vatablus; "obtegumentum cordis", Stockius, p. 199. so Ben Melech; "scutum cordis", V. L. "clypeum cordis", Munster.