Reveal a secret especially carelessly or by mistake
A similar metaphorical use of bag may be found in the French phrase vider le sac literally empty the bag meaning tell the whole story.
1996 - Bernard Connolly - The Rotten Heart of Europe Tim Renton... at odds with his leader on Europe, let the cat out of the bag when he told a television audience….we need a strong Europe to maintain our independence from the United States and the Pacific Rim.
Related Idioms :
all cats are grey in the dark
The Qualities that distinguish people from one another are obscured in some circumstances and if they can't be perceived they don't matter. – proverb
The US version of this proverb is at night all cats are gray.
bell the cat
take the danger of a shared enterprise upon yourself
Bell the cat alludes to the fable in which mice or rats have the idea of hanging a bell around the cat's neck so as to have warning of its approach, the only difficulty being to find one of their number willing to undertake the task.
the cat has got his tongue
Someone is remaining silent.
a cat may look at a king
even a person of low status or importance has rights – proverb
1998 - Times - A cat may look at a king. The cat may be wrong in its conclusions, but others, following its gaze, can draw their own.
a dead cat bounce
a misleading sign of vitality in something that is really moribund – informal
A dead cat might bounce if it is dropped from a great height…the fact of it bouncing does not reliably indicate that the cat is alive after all. The expression was coined in the late 20th century by Wall Street traders to refer to a situation in which a stock or company on a long-term, irrevocable downward trend suddenly shows a small temporary improvement.
enough to make a cat laugh
extremely ridiculous or ironic – informal
This expression dates from the mid 19th century and is associated with the story of Puss in Boots.
fight like cat and dog
(of two people) be continually arguing with one another.
1995 - Edward Toman - Dancing in Limbo - Her desertion of him hadn't come as a total surprise... for the pair of them had been fighting like cat and dog for the best part of a year.