He is a sluggard, wise in his own eyes, won't listen to rebuke or even painful punishment, repeats folly, meddles in quarrels not his, speaks nicely but with hatred underneath.
Folly flies away and is of little consequence (vs 2), though it can do plenty damage (vs 21, 10).
Jesus calls those who don't do His words foolish (Matt 7:26). The Pharisees are fools for their focus on outward cleanliness while their hearts are defiled (Matt. 23:17). The man who builds bigger barns to take life easy is a fool (Luke 12:20). He even calls the Emmaus road disciples foolish for being slow to believe Him after the resurrection (Luke 24:25). But we may not call someone a fool out of anger or insult or hatred (Matt. 5:22).