Jeremiah 17:9

The heart is deceitful - עקב הלב akob halleb, "the heart is supplanting - tortuous - full of windings - insidious;" lying ever at the catch; striving to avail itself of every favorable circumstance to gratify its propensities to pride, ambition, evil desire, and corruption of all kinds.

And desperately wicked - ואנש הוא veanush hu, and is wretched, or feeble; distressed beyond all things, in consequence of the wickedness that is in it. I am quite of Mr. Parkhurst's opinion, that this word is here badly translated as אנש anash is never used in Scripture to denote wickedness of any kind. My old MS. Bible translates thus: - Schrewid is the herte of a man: and unserchable: who schal knowen it?

Who can know it? - It even hides itself from itself; so that its owner does not know it. A corrupt heart is the worst enemy the fallen creature can have; it is full of evil devices, - of deceit, of folly, and abomination, and its owner knows not what is in him till it boils over, and is often past remedy before the evil is perceived. Therefore trust not in man whose purposes are continually changing, and who is actuated only by motives of self-interest.

The train of thought is apparently this: If the man is so blessed Jeremiah 17:7-8 who trusts in Yahweh, what is the reason why men so generally "make flesh their arm"? And the answer is: Because man's heart is incapable of seeing things in a straightforward manner, but is full of shrewd guile, and ever seeking to overreach others.

Desperately wicked - Rather, mortally sick.

17:9 The heart - There is nothing so false and deceitful as the heart of man; deceitful in its apprehensions of things, in the hopes and promises which it nourishes, in the assurances that it gives us; unsearchable by others, deceitful with reference to ourselves, and abominably wicked, so that neither can a man know his own heart, nor can any other know that of his neighbour's.