“The heart.” This is the only time in the Hebrew text where “heart” is combined with the definite article, and “the heart” is the heart of anyone, that is to say, people in general. The human heart is deceitful.
“deceitful above all things, and is incurable.” The human heart is deceitful, and is incurable. The Hebrew word translated as “incurable” generally refers to being sick, and especially sick beyond being able to be healed (cp. ESV: “desperately sick”). Here that meaning applies, but also by extension it refers to being sick in the sense that it is “corrupt,” “depraved,” “wicked,” etc., and thus “incurably bad” or “desperately corrupt” (cp. ASV; GNV; KJV; NET; NJB; NLT; NRSV; RSV). Because humans have a sin nature that will never go away in this life, there is a very real sense in which the natural human heart is indeed “incurably bad” (NET).
That people are born with a deceitful, corrupt heart shows up in many areas of life. It explains why children have to be taught civilized characteristics such as sharing with others, no biting or hitting, no interrupting the conversation of others, etc. Those things do not come “naturally.” It also explains why even godly people fight with being selfish and self-centered throughout their life, and why the majority of the people of earth refuse to humble themselves to God and obey Him and His Son.
Thankfully, the human heart is constantly changing, and the wise person is in a constant dialogue with their heart so that it conforms to God’s ways of thinking and acting. The more we obey God despite our natural inclinations, the more our heart conforms to God’s ways, and the more “natural” it becomes to think and act like God (a heart changed to God’s ways is also sometimes referred to as “an educated conscience,” that is, a conscience educated to think like God).
The Devil knows that the human heart is deceitful and sick (corrupt), so he keeps up a constant societal pressure for people to “follow the heart,” because he knows it often leads them astray. “Follow your heart” is not the message of Scripture. The message of Scripture is “obey God.” As Moses wrote long ago, “It will be our righteousness if we are careful to do every one of these commandments before Yahweh our God, as he has commanded us” (Deut. 6:25).
“who can know it.” The Hebrew word is the common word yada, “to know,” (#03045 ידע), and it has a wide range of meaning including to know and to understand, and both meanings are applicable here. Who can really “know” their heart and fathom how deceitful and corrupt it really is and who can “understand” their heart and why it causes a person to think and act the way that they do? The human heart is corrupt, which is why we cannot “trust our heart,” we have to trust what God says.