“sense.” The Hebrew word is leb (#03820 לֵב), which is often translated “heart,” but this is one of those cases where that translation would cause confusion. In modern English, the word “heart” usually refers to emotion or passion, but that is not its meaning here. The function of the brain was unknown in biblical times, so things that we generally assign to the brain, like thinking, attitudes, understanding, and good sense, were assigned to the heart. In this case, fools die because they lack “sense,” or good sense. In this case, the fools not only die on this earth, they experience everlasting death because they are so foolish they never get saved and gain everlasting life.
[For more on the Hebrew word leb and “heart,” see commentary on Prov. 15:21, “sense.”]