let the cat out of the bag
泄露秘密，意外地将某事公之于众
口语·意外泄密
英文释义
to reveal a secret or disclose information that was intended to be kept secret
例句
I was planning a surprise party, but my brother let the cat out of the bag.
典故来源
源自中世纪市场，骗子将猫冒充小猪装入袋子出售，若猫跑出则骗局败露。