Jeremiah 17:9 - Matthew Poole's English Annotations on the Holy Bible

The words translated deceitful, and desperately wicked, are very variously translated, fraudulent, perverse, supplanting. He speaks to the Jews, that they might not lean too much to their own counsels, fancies, or understandings; but it is a proposition true concerning the hearts of all the sons and daughters of men; 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 nourisheth, in the assurances that it gives us, &c.; unsearchable by others, deceitful with reference to ourselves, and abominably wicked, so that neither can a man know his own heart, neither can any other know our hearts.