Quick answer
Wisdom spent on a scoffing hearer is not received, it is despised. The saying counsels timing and audience: some ears turn good words into objects of contempt, and wisdom includes knowing when to stop offering.
Read both lines
The command is startling from a book devoted to teaching: stop talking. The reason clarifies it. The fool in view does not merely miss the wisdom of your words; he "despises" it, treats it with active contempt, and every additional sentence gives him more to sneer at. The saying is close kin to 9:7–8, where correcting a scoffer earns you harm, and it anticipates Jesus' hard proverb about pearls and swine (Matthew 7:6). None of these teach contempt for the fool; they teach reverence for wisdom. Words of substance are not to be poured out where their only fate is mockery. Silence, here, is not giving up on the person; it is refusing to feed the scorn, and sometimes it is the only message a scoffer can still receive.
Related: Proverbs 9:7–8 · Proverbs 26:4–5 · Matthew 7:6
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.