To "let the cat out of the bag" means to accidentally reveal information that was meant to be kept secret. It is always unintentional — you do not let the cat out of the bag on purpose. A surprise party is ruined if someone lets the cat out of the bag before the guests arrive.
The phrase connects to a form of market deception common across medieval Europe. Buyers purchasing a young pig at market would often receive the animal in a bag or sack (called a "poke" — hence the related phrase "a pig in a poke," meaning to buy something without inspecting it). Dishonest sellers would sometimes substitute a cat — roughly similar in weight to a small piglet — for the pig. A buyer who checked the bag before leaving would expose the fraud. The cat — literally — would be let out of the bag.
The companion phrase "pig in a poke" is attested in English from the 16th century. "Let the cat out of the bag" appears in print by the mid-18th century — a 1760 publication uses it in close to its modern figurative sense.
"Never buy a pig in a poke" means never commit to something you have not properly examined or verified. The same market-fraud scenario produced both phrases, giving English two complementary idioms about the same act of deception — one from the buyer's perspective, one from the moment of discovery.
The same image appears in other European languages. German has "die Katze im Sack kaufen" (to buy the cat in the sack). French has "acheter un chat en poche" (to buy a cat in a pocket). The shared origin likely reflects a form of fraud that was genuinely widespread across European markets.
Both mean revealing a secret, but 'spill the beans' can be slightly more deliberate while 'let the cat out of the bag' emphasises accidental disclosure. In practice, they are often used interchangeably.
A pig in a poke means buying something without inspecting it first — committing to a purchase sight unseen. It comes from the same marketplace context as 'let the cat out of the bag.'