The proverb means that good cannot comes out of evil. Grapes are a kind of good fruit. But this fruit will not come out of thistles. Thistles are thorny plants. So they stand for evil, grapes stand for good. So the proverb means that good can never come out of evil. Good can come only from good. Only evil will come from evil. Only a good man can spread the message of love, sympathy and concern. He will be an asset to the society. A bad man spreads hatred and apathy. He is an ulcer to the society. He spreads the false message that anything is permissible to make money. Thus such a man can mislead the society. So the society does not expect any good from him. Good actions cannot be expected from such a person, wicked people may make money illegally. But their prosperity is only temporary. They will have no happiness in life. Thus we see that no good can come out of evil deeds. Good can come only out of good. It is permanent, too.