In the Warring States Period, a man in the state of Chu offered a sacrifice to his ancestors. After the ceremony, he gave a beaker of wine to his servants. The servants thought there was not enough wine for all of them, so they decided that each would draw a picture of a snake; the one who finished first would get the wine.
One servant drew very rapidly. Seeing the others were still busy, he added feet to his snake. At that moment, another man finished, snatched the beaker, and drank the wine, saying, "A snake doesn't have feet. How can you add feet to a snake?"
This idiom refers to ruining something by doing unnecessary and surplus things.