The story
A poke was a small sack, and the old warning against buying “a pig in a poke” was simple: look before you pay. English proverbs were giving that advice as far back as the 1200s. The word poke lives on in its little cousin, pocket.
One popular story says dishonest sellers slipped a cat into the sack in place of a piglet. Open the bag, and out jumped the cat—and the secret. It is a tidy tale, and it gets told often.
But no one has found evidence that such a trick was ever common, and the earliest uses of the phrase have nothing to do with markets. Word historians admit the real picture behind it has been lost.
What is certain is the timing. “Let the cat out of the bag” was in print by 1760 and turned up again and again through that decade, as if everyone had just started saying it.
