fool: the dull, smug person who is settled in his folly, closed to correction, and trusts only himself
This is the dominant word for a fool in the wisdom books. Nearly all of its roughly 70 occurrences sit in Proverbs and Ecclesiastes, with three appearances in wisdom psalms. It does not describe someone of low intelligence or limited education. It describes a settled disposition: the person who is satisfied with his own thinking, closed to correction, and sees no need for anyone to teach him. The root connection to כֶּסֶל (loins, flank) suggests the figurative path of "fat at the loins" to "sluggish" to "dull", though scholars debate this.
The fool in Proverbs is the standing counterpart to the wise person, deployed as a constant pedagogical warning. He talks too much and at the wrong moment, blurting out whatever he feels and broadcasting his own folly. He despises instruction and resents correction, so he circles back to the same mistake like a dog returning to what it left behind. He is dangerous company. Proverbs 26 gathers its sharpest fool-sayings around him, painting an almost satirical picture of a person whose self-assurance is his undoing.
In Ecclesiastes the same vocabulary appears in a different frame. Qoheleth uses the word to draw the Proverbs contrast, the wise man has eyes in his head while the fool walks in darkness (2:14), then dismantles the contrast by noting that one fate overtakes them both. Death levels what wisdom sought to distinguish. Fox and Longman both read this as Qoheleth's deliberate reuse of the inherited wisdom idiom to expose its limits. The character of the fool is unchanged; what changes is the evaluative conclusion drawn about him.
In the three wisdom psalms (49, 92, 94), this word appears alongside בַּעַר ("brutish"), the term for animal-like senselessness. That pairing reinforces the center of gravity: not willful rebellion so much as thick, stubborn obtuseness, the refusal to see what is plainly in front of one.