What does Proverbs 26:8 mean?

The purpose of placing a stone in a sling is making it ready to be thrown. If the stone is tangled in the sling, or tied in so it cannot come out, it defeats the purpose of the sling. Not only is such a thing pointless, but it can also be dangerous and wasteful. In the same way, it's senseless to give authority, power (Proverbs 26:1), or trust to someone lacking godly common sense. At best, they might fail and accomplish little. At worst, that choice may lead to harm or unintended consequences. It can certainly lead others to question the sense of the person who gives "honor" to a fool.

Another interpretation suggested by some commentators is that the stone placed in the sling is soon thrown and lost. That is, putting a stone in a sling is temporary; the sling will naturally throw the stone away. In a comparable way, giving trust or power to a fool can only be temporary; they will naturally cast that honor aside.

History is filled with examples of foolish people coming into power and creating great harm for those around them (Proverbs 29:2).