Sometimes a market trader would put a cat in his bag and pretend it was a pig to fool the customer! So if someone lets the cat out of the bag, the secret would be open to us.

有时一个市场商人会把一只猫放在他的包里，假装它是一只猪来愚弄顾客！所以如果有人让猫从包里出来，秘密就会为我们所知。

youdao

The milk you put out in the saucer was quickly lapped up by the cat.

你倒在碟子里的牛奶很快就被猫舔光了。

《新英汉大辞典》

Their engagement was to be a secret but the ring on her finger let the cat out of the bag.

他们俩订婚原本想保密的，可是她手指上的戒指却把这个秘密泄露出来了。

youdao

If you tell about something that was supposed to be a secret, we say you let the cat out of the bag.

VOA: special.2009.09.27

As it turned out, her mother made a miraculous recovery, but the cat was already out of the bag.

NEWYORKER: Temporary

The court heard it was made out of chicken wire, and as the cat entered a peg was dislodged, sealing the cage with a plastic flap.

BBC: Pensioner Gordon Wilson trapped neighbour's cats in cage

"I have a colleague at London Zoo who was called out last year after the police spotted a 'big cat', only to find it was an ordinary ginger moggy, " he said.

BBC: News | UK | Home Counties puma alert