Closes the fool run, and it is the chapter's hinge. `בְּעֵינָיו`, "in his own eyes," is the phrase that ties 26:5, 26:12 and 26:16 together.
Quick answer
After eleven verses of contempt for the fool, the chapter says there is somebody worse. The man who thinks he is wise is further gone.
Read both lines
The move is deliberate and it lands hard because the chapter earned it. Eleven verses have established the fool as useless, dangerous, and unteachable, and then the twelfth says a different man has less hope. That is not rhetorical excess; it is a claim about teachability. A fool might still discover he is one. A man certain of his own wisdom has closed the only door correction comes through.
The chapter has already used this phrase at verse 5, where the danger of leaving a fool unanswered was that he would be wise in his own eyes, and it will use it again at verse 16 for the sluggard. Chapter 3 stated the whole thing as a command: "Do not be wise in your own eyes; fear the LORD and depart from evil" (3:7). This verse is what happens when that command is ignored.
Related: Proverbs 3:7 · Proverbs 26:5 · Proverbs 26:16 · Proverbs 28:26
New King James Version®, Copyright © 1982, Thomas Nelson. All rights reserved. Full copyright information. Scripture provided by API.Bible.
Go Deeper
If this verse has been used to put weight on you that it was never carrying, ask about it directly.