Charles Ellicott Commentary Jeremiah 17:9

"The heart is deceitful above all things, and it is exceedingly corrupt: who can know it?" — Jeremiah 17:9 (ASV)

The heart is deceitful ... —The sequence of ideas seems to be as follows: If the blessing and the curse are therefore so plainly marked, how is it that man chooses the curse and not the blessing, the portion of the heath in the desert rather than that of the tree planted by the waters? And the answer is found in the inscrutable self-deceit of his nature, blinding his perceptions of good and evil.

Desperately wicked. —Rather, incurably diseased, as in Jeremiah 15:18, Jeremiah 30:12, Jeremiah 30:15, Isaiah 17:11, and elsewhere. Wickedness is, of course, implied, but it is regarded rather as a moral taint following on the deliberate choice, than as the choice itself.