It means to reveal a secret. This phrase comes from the trick played on unsuspecting purchasers of sucking pigs at old English country fairs. The pig was shown to the buyer, then put in a sack. While the deal was finalized a sack with a less valuable 'cat' was quickly substituted, and this was what the buyer took away. When he opened the sack, he 'let the cat out of the bag' and the subterfuge was revealed. This saying is known since 1760.

Übersetzung:

die Katze aus dem Sack lassen

Gegenteil(e)

The opposite of "to let the cat out of the bag" is "to dissimulate something".

Synonym(e):

You could also say: to reveal a secret