let the cat out of the bag字面意思放出袋中猫，真实含义是泄密，说漏嘴，(无意中)泄露秘密。

【例句】
I wanted it to be a surprise, but my sister let the cat out of the bag.
我想给大家来个惊奇，可我妹妹却先说漏了嘴。

Whatever you do, just don't let the cat out of the bag!
不管你干什么，只是不要泄露秘密！