【成語來源】
The affair of the east window is exposed./The cat is out of the bag。
東窗事發
During the final years of the Song dynasty (sòng cháo 宋朝)， China was invaded by Jin (jīn金) people from the north.
宋朝末年，北方的金人來侵犯。
General Yue Fei led the army in resistance, and defeated the enemy.
將軍岳飛率領軍隊勇敢抵抗，終於把敵人打敗。
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.
正想乘勝追擊敵人的時候，忽然皇帝下了十二道金牌，命令岳飛和軍隊立刻回去。
As it turned out, this was actually a plot of Chin Hui, a Sung dynasty official who was conspiring with the order to frame Yue Fei, Chin Hui had tricked him into returning early.
原來這是私通敵人的太師秦檜的詭計，假冒皇帝的意思來陷害岳飛。
Chin Hui had thought this plot up together with his wife, under the east window of their house.
秦檜的這些詭計，都是他和妻子王氏在家裏的東窗下祕密商量出來的。
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.
後來秦檜和他的兒子秦僖相繼死亡，王氏請了幾個研究神鬼靈魂的人來，想知道他們父子死後的情形。
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."
來的人在做法以後告訴王氏説：“我先見到了公子秦僖，他告 訴我秦太師在酆都城，我到了酆都城，看見太師正戴着鐵枷做苦工呢。他讓我轉告你，你們在東窗下商量的那件事已經被揭發了。”
Now, when a wrong-doing is brought to light, it may be said that "the affair of the east window has been exposed."
現在當一件醜事被曝光之後，就可以説是：“東窗事發”了。
【中外差異】
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.
英語中的與“東窗事發”意義相近的是“the cat is out of the bag”，就是“貓從袋子裏走出來了”，可以把貓想象成“祕密”，把不為人知的祕密藏在袋子裏，貓一旦從袋子中走出，祕密也就隨之公開了，也就是“東窗事發”。
【成語活用】
I've been keeping that thing a family now the cat is out of the bag because of you!
我一直把那件事當做家族的祕密，但如今卻因為你而東窗事發！
學問齋