In the Spring and Autumn Period, a man in the state of Song raised monkeys who could understand his words. As he grew poor, he decided to reduce their food. He first proposed giving them three acorns in the morning and four in the evening. The monkeys protested angrily. The man then asked, "How about four in the morning and three in the evening?" The monkeys were satisfied.
This idiom originally described deceiving others with a trick. Later, it came to mean being fickle or constantly changing one's mind.