A thorn is a painful thing to endure. If one pierces your hand, you will know it instantly and you will learn from it. Over the years of having this happen to me, I've learned which trees, bushes, and flowers have thorns. I either avoid them - or act with great caution when I get around them. A rose is a beautiful thing to see - and very pleasant to smell - but if you are unaware of the issue of the thorns - it can also be a very unpleasant thing to grasp in the hand. Thorns are given in life to teach us - and to warn us. We learn from them to be cautious and careful.
This proverb tells us that a proverb in the mouth of a fool is about as profitable as a thorn in the hand of a drunkard. A drunk is in a deadened state. If a thorn were to fall into his hand, he might not even know about it until the following morning when he sobers to find a wound in his hand. A drunken fool will be more likely to injure himself in a thorn bush and not really heed the warning of the thorns. He is too drunk to realize that the bush is warning him to stop and get away. Therefore he injures himself worse - and ignores the danger of the situation.
A fool with a proverb is the same way. He may have a proverb right at the tip of his tongue. But though he speaks it - he learns nothing from it. He does not heed the warning - and may even mock it. He does not follow the advice of the God who gave it for his blessing and wisdom. Therefore it profits him nothing.
If you want to be wise - you will have to be able to heed the warnings of God's Word. You will want to avoid what the Lord tells you will harm you. Those warnings are like thorns. We are told some very valuable information that uses this same kind of picture by Solomon in the book of Ecclesiastes.
"In addition to being a wise man, the Preacher also taught the people knowledge; and he pondered, searched out and arranged many proverbs. The Preacher sought to find delightful words and to write words of truth correctly. The words of wise men are like goads, and masters of these collections are like well-driven nails; they are given by one Shepherd." (Ecclesiastes 12:9-11, NASB)
Solomon tells us that there was one who taught the people knowledge through proverbs he wrote. Later he calls these proverbs and sayings goads. A goad is a sharp instrument that was used to teach oxen how to plow without kicking at their master. When they would kick at him - their leg would run into a sharp object that would poke them. A well trained ox would never kick or buck at the direction of the farmer because the goad would have trained him that to do so was foolish - and painful.
Proverbs are meant to be goads - thorns if you will. They are meant to train us to walk in wise ways. They warn us against painful moments that will come if we continue to walk as a fool in this life. May God give us grace to be the kind of person to whom a proverb would be profitable - and not like a drunkard who has a thorn fall into his hand - and doesn't even feel it.