During the final years of the Song dynasty (sòng cháo 宋朝), China was invaded by Jin (jīn金) people from the north.
Just as he was about to take advantage of his victory and attack the Jin army, he received word that the emperor had ordered him to return.
As it turned out, this was actually a plot of Chin Hui, a Sung dynasty official who was conspiring with the enemy.In order to frame Yue Fei, Chin Hui had tricked him into returning early.
Chin Hui had thought this plot up together with his wife, under the east window of their house.
Sometime later, Chin Hui and his son Chin Shi died, one right after the other. Chin Hui's wife hired someone who could communicate with ghosts and spirits to come and find out how her husband and son were doing after death.
After he had finished, the man said to her, "I first went to see your son, who told me that your husband was in the city of Feng-du. So I went there and found him doing hard labor, wearing iron cangue. He asked me to tell you that the thing which you two discussed under the east window has already been exposed."
Now, when a wrong-doing is brought to light, it may be said that "the affair of the east window has been exposed."
A secret is like a cat that has been kept in a bag. Once you let the cat out of the bag, it goes wherever it wants.
英语中的与“东窗事发”意义相近的是“the cat is out of the bag”,就是“猫从袋子里走出来了”，可以把猫想象成“秘密”，把不为人知的秘密藏在袋子里，猫一旦从袋子中走出，秘密也就随之公开了，也就是“东窗事发”。
I've been keeping that thing a family secret.But now the cat is out of the bag because of you!