6 Like cutting off one's feet or drinking violence is the sending of a message by the hand of a fool.
7 Like a lame man's legs that hang limp is a proverb in the mouth of a fool.
8 Like tying a stone in a sling is the giving of honor to a fool.
9 Like a thornbush in a drunkard's hand is a proverb in the mouth of a fool.
These four verses have something in common . . . Solomon is saying that these things "don't add up." In other words, they don't make sense.
First of all, you would not send an important or confidential message by entrusting it to someone who you knew was foolish, would you? They might not value the importance of the message, and dilly-dally instead of hurrying to deliver it for you. They might also tell other people your confidential information, instead of keeping it secret.
Next, a proverb, or wise saying, is useless in the mouth of a foolish person --- they will more than likely turn it into a jest or joke, and make fun of it. So the wise words become useless, like the lame man's legs which he cannot use to walk or run.
We've already talked about giving honor to a foolish person --- Solomon says here that it would be like tying a stone in a sling. Remember David's sling that he put the stone into, and then used the sling to hurl the rock at Goliath and kill him? Well, what do you think would happen if you put the stone into the sling and then tied it shut? Yep, you're right --- nothing would happen, because the rock couldn't come out of the sling!
Lastly, he mentions again that a proverb in a foolish person's mouth is like a thorn in a drunkard's hand: it hurts him and he tries to get rid of it as soon as he can, by flailing around and perhaps harming others.
It occurred to me as I studied these verses, that we need to make sure that our lives "add up." If we say we are Christians, there will be those who quietly, carefully watch us. Do our lives show that we are "walking the walk," or that we are just "talking the talk?" Are we an example in what we say and do?
Do the things we say, and the things that we do, add up?