C. F. Keil and Franz Delitzsch on Jeremiah 17:9

Jer 17:9

To bring this truth home to the people, the prophet in Jer 17:9 discloses the nature of the human heart, and then shows in Jer 17:10 how God, as the Searcher of hearts, requites man according to his conduct. Trust in man has its seat in the heart, which seeks thereby to secure to itself success and prosperity. But the heart of man is more deceitful, cunning than all else עקב, from the denom. עקב .moned , to deal treacherously). אנוּשׁ, lit., dangerously sick, incurable, cf. Jer 15:18; here, sore wounded by sin, corrupt or depraved. Who can know it? i.e., fathom its nature and corruptness. Therefore a man must not trust the suggestions and illusions of his own heart.