“Let the cat out of the bag” is recorded in print by 1760, when it already meant to reveal something meant to stay hidden. That early appearance tells us the expression was established enough to be understood, but it does not identify who first said it or what incident inspired it. Its image is vivid: something concealed in a bag escapes into view, and the secret can no longer be kept.
The best-known origin story links the phrase to medieval markets. In this version, a dishonest seller passes off a cat as a valuable piglet by hiding it in a bag; once the buyer opens the bag, the trick is exposed. The tale is often connected to the older expression “buy a pig in a poke,” meaning to buy something without inspecting it. That connection makes an appealing story, but the existence of shady market deals does not prove that the cat phrase began this way. There is no reliable evidence tying the expression to a specific custom of substituting cats for pigs.
What can be traced more confidently is the phrase’s meaning. By the eighteenth century, the escaped cat served as a compact image for a secret or surprise coming to light. Over time, the expression settled into everyday English, often used when someone reveals information accidentally, before the right moment, or simply sooner than planned. It can refer to anything from a surprise party to a business decision, and it does not necessarily imply deception.
“You’ve let the cat out of the bag about Maya’s promotion,” Luis whispered after she mentioned the new office. “We planned to announce the merger next week, but the press release let the cat out of the bag early.” In both cases, the phrase captures the instant when private information becomes public, whether through a careless comment or an official announcement.
The medieval-market explanation may be memorable, but the evidence leaves the exact origin uncertain. The phrase endures because its image is playful and immediate: once the cat is out, there is no easy way to put the secret back.