let the cat out of the bag (无意中)泄露秘密;说漏嘴

【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。

Reveal information; let the cat out of the bag.
把信息泄露出去；泄露秘密。