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

Letting the cat out of the bag is supposed to stem from a trick played in old markets where buyers were duped when they thought they were buying a Suckling pig.

一般认为，“把猫从袋子里放出来”的说法源自古代集市上的一个把戏：有时，商贩会欺骗买家，在他们购买乳猪的袋子里装一只猫。

youdao

They had a large canvas bag, which tied up at the mouth with strings: into this they slipped the guinea-pig, head first, and then sat upon it.

他们用一只大帆布袋，把那只胆鼠头朝里塞进去，用绳扎上了袋口，然后他们坐在袋上。

youdao

The woman asked to see it. When the dishonest tradesman opened the bag, out jumped a cat C not a pig.

VOA: special.2009.01.25