1
general::
Phrase(s): let the cat out of the bag
Fig. to reveal a secret or a surprise by accident. • When Bill glanced at the door, he let the cat out of the bag. We knew then that he was expecting someone to arrive. • It’s a secret. Try not to let the cat out of the bag.
McGrawhill's American Idioms And Phrasal Verbs
2
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Give away a secret, as in Mom let the cat out of the bag and told us Karen was engaged. This expression alludes to the dishonest practice of a merchant substituting a worthless cat for a valuable pig, which is discovered only when the buyer gets home and opens the bag. [Mid-1700s] Also see PIG IN A POKE.
American Heritage Idioms