let the cat out of the bag的意思是(无意中) 泄露秘密；说漏嘴；泄露机密。

【例句】
I had let the cat out of the bag, so to speak.
我可以说是无意中泄漏了秘密。

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。