Le Bu Si Chu (To Have So Much Fun That One Forgets About Shu)
Liu Shan (刘禅), son of Liu Bei (刘备), was ruler of the kingdom of Shu Han during the Three Kingdoms Period (220-280AD). He was a coward who cared only about amusing himself and knew nothing about ruling a country.
One time, the kingdom of Wei attacked Shu and took Liu Shan prisoner.
The people of Wei gave Liu Shan the title "Duke of peace and Happiness", and took him to live in their capital. Liu Shan had so much fun in the Wei capital that he soon forgot he was the ruler of a defeated kingdom. One day, Sima Zhao, a very powerful Wei general, invited Liu Shan over to dinner. Sima Zhao intenetionally hired some people to perform traditional Shu songs and plays at dinner, in order to insult Liu Shan. When Liu Shan's subordinates heard the music and saw the plays of their homeland, they grew very sad. Only Liu Shan was completely unaffected, and continued drinking wine and being merry. When Sima Zhao asked him,"Don't you miss Shu?" Liu Shan laughed and replied, "I'm having so much fun here; of course I don't miss Shu." And so Sima Zhao had even less respect for Liu Shan.
Today, when a person is enjoying himself so much that he forgets about his home and his work, we can say that he "is having so much fun that he has forgotten about Shu."