heart. Put by Figure of speech Metonymy (of Subject), for courage.
king... princes... priests... prophets. All had become false and corrupt since Josiah's day.
Related Commentaries of Jeremiah 4:9
Jeremiah 4:9
9 And it shall come to pass at that day, saith the LORD, that the heart of the king shall perish, and the heart of the princes; and the priests shall be astonished, and the prophets shall wonder.