What does Proverbs 29:9 mean?

It is tempting to bicker with an irrational, unfair, immoral person. One feels a natural desire to silence a fool, if for no other reason than to stop hearing their nonsense (Proverbs 27:3). That's not always a good idea (Proverbs 26:4), nor is it always possible. Some "fools" are deliberately closed to reason; they bicker and argue because they enjoy the attention and feeling of being in control. Such are the "scoffers" mentioned in other proverbs (Proverbs 9:7; 29:8). They are not interested in truth, and may not even care about winning the argument, but only love tricking others into playing their games. Arguing reasonably may only make them angry (Matthew 7:6) and would be a waste of time (Titus 3:9–11).

Worse is when the "fool" successfully baits someone into revenge or overreaction. David was enraged by Nabal—whose name literally means fool—but Nabal's wife persuaded David not to retaliate (1 Samuel 25:25–26). She brought the supplies David had requested and advised him that he would "have no cause of grief or pangs of conscience for having shed blood without cause or for my lord working salvation himself" (1 Samuel 25:31). David replied, in part, "Blessed be the Lᴏʀᴅ, the God of Israel, who sent you this day to meet me! Blessed be your discretion, and blessed be you, who have kept me this day from bloodguilt and from working salvation with my own hand!" (1 Samuel 25:32–33).