fool: the person who simply has no sense, whose emptiness of judgment shows up in what he does and how much he talks
This is the rarest of the Hebrew words for a fool. It occurs seven times in all, four of them in Ecclesiastes and two in Jeremiah. Where the common wisdom word for a fool (כְּסִיל) names a settled, self-satisfied person who has shut out correction, this word points at something plainer: the sense is simply missing, and the shortage becomes public. Ecclesiastes says his heart, which in Hebrew is the seat of judgment rather than of feeling, is lacking, and that he announces what he is to everyone he passes on the road (Ecclesiastes 10:3).
Ecclesiastes does most of the work with this word, and always in the same frame, the fool set beside the wise person. Nobody knows whether the heir who inherits a life's labor will be wise or a fool (Ecclesiastes 2:19). Do not be one, the book warns, or you will die before your time (Ecclesiastes 7:17). He piles up words about a future that no one can see (Ecclesiastes 10:14). The noun from the same root (סִכְלוּת) names folly as a quality; this word names the person who is full of it.
Jeremiah turns it on a whole nation. The people are "senseless children" who do not know God (Jeremiah 4:22), a "foolish people" with eyes that do not see and ears that do not hear (Jeremiah 5:21). Nothing is being said there about anyone's intelligence. The charge is that a people who had every means of knowing God had lost the sense to use them, which is why the prophet stacks this word next to the coarser fool-word (אֱוִיל) in the same breath.