John Calvin Commentary Lamentations 3:65

"Thou wilt give them hardness of heart, thy curse unto them." — Lamentations 3:65 (ASV)

He expresses what the vengeance was to be, namely that God would give them up to a reprobate mind. For by מגנת-לב, meganet-leb, he no doubt meant the blindness of the heart, and at the same time included stupidity, as though he had said, “O Lord, so oppress them with evils, that they may become stupefied.” For it is an extreme evil when we are so overpowered that we are not, as it were, ourselves, and when our evils do not drive us to prayer.

We now perceive what the Prophet meant by asking God to give his enemies the impediment of heart—namely, that God might take away their sound mind and smite them with blindness and madness, as it is said elsewhere. — I am moving on quickly to finish, lest the hour prevent us. The last verse of this triple alphabet follows in the next passage.