Let the cat out of the bag — To accidentally reveal a secret, often causing embarrassment or spoiling a surprise that was meant to be kept hidden.
Meaning
When someone “lets the cat out of the bag”, they accidentally disclose information that was supposed to remain secret. The idiom nearly always implies that the disclosure was unintentional — the speaker did not mean to reveal the information, but it slipped out in conversation, through a careless comment, or simply by saying too much. The result is often a spoiled surprise, a leaked plan, or an embarrassing revelation.
This is one of the most common English idioms for talking about secrets and is firmly informal in register. You will hear it in everyday speech, in television dramas, and in casual journalism. It is rarely used in formal writing. Native speakers treat it as completely natural; for learners, it typically appears at B1 level and is well worth adding to your active vocabulary.
Origin & History
The most widely accepted origin links the phrase to a type of market fraud common in medieval Europe. A seller would offer a piglet for sale inside a tied sack — a common way of transporting small livestock at the time. A dishonest trader might substitute the piglet with a cat, which was far less valuable. If the buyer opened the sack at the market before completing the purchase, the trick was exposed. “Letting the cat out of the bag” therefore came to mean revealing a deception or a secret that someone had tried to conceal.
The phrase was first recorded in English print in the 18th century, and a related German expression, die Katze im Sack kaufen (“to buy a cat in a sack”, meaning to buy something without inspecting it first), suggests the market-fraud story has genuinely old roots across Northern Europe. By the 19th century the idiom had fully shifted from its literal meaning to its modern figurative sense of accidentally revealing any kind of secret, not just a commercial swindle.
Example Sentences
| Sentence | Context |
|---|---|
| She let the cat out of the bag about the surprise party. | Accidentally spoiling a planned surprise |
| The junior manager let the cat out of the bag when he mentioned the redundancies in an all-hands meeting. | Premature disclosure of sensitive workplace news |
| I nearly let the cat out of the bag when I bought the tickets — I had to pretend they were for something else entirely. | Narrowly avoiding revealing a secret |
How to Use It
This idiom is informal and suits spoken English and casual written contexts such as personal emails, text messages, blog posts, and social media. It would sound out of place in a formal business report, an academic essay, or official correspondence. The idiom is most naturally used in the past tense (“she let the cat out of the bag”) or in a warning construction (“don’t let the cat out of the bag”). It collocates naturally with “about”: you let the cat out of the bag about something.
- Always use the definite article: the bag and the cat are fixed — never say “a bag” or omit either article.
- The idiom describes an accidental disclosure; if someone reveals a secret deliberately, it is more natural to say “spill the beans” or “give the game away”.
- Be careful with tense: use the past simple for a completed disclosure (“he let the cat out of the bag”) and a modal or negative imperative for warnings (“don’t let the cat out of the bag”).
Common Mistakes
Mistakes to Avoid
She let the cat out of a bag about the party.
She let the cat out of the bag about the party. — The article ‘the’ before ‘bag’ is fixed and cannot be changed.
He let out the cat from the bag by accident.
He let the cat out of the bag by accident. — The word order in idioms is fixed; do not rearrange the phrase.
I let the cat out of the bag intentionally to upset him.
I spilled the beans intentionally to upset him. — This idiom implies an accidental disclosure; for deliberate revelations choose a different expression.
Similar Idioms
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