Part of the fool run. Verse 9 makes the same comparison with a different image.
Quick answer
A proverb quoted by a fool is a leg that does not carry weight. The apparatus is there and it does nothing.
Read both lines
The comparison is about function rather than appearance, and it is uncomfortable enough that modern readers may wince at the image. Legs that hang are present, correctly formed, and useless for the one thing legs are for. That is the claim about a proverb in the wrong mouth: the words are right, the shape is right, and it will not bear weight, because a proverb only works when the person deploying it understands the situation it fits.
This chapter is unusually self-aware, since a book of proverbs is here warning that its own contents can be repeated by people who do not grasp them. Verse 9 will say it again with a thorn.
Related: Proverbs 26:9 · Proverbs 17:16 · Proverbs 15:2
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.