Let the cat out of the bag is a common saying that means to make a secret known.
This expression probably dates back to the Middle Ages when unscrupulous vendors in the marketplace would sometimes substitute a cat when someone bought a piglet, putting the cat in a bag where its thrashing would look like the struggle of a pig. Once the buyer walked away, they would let the cat out of the bag, revealing the trickery.
The second theory has to do with a multi-tailed whip that was called a “cat o’ nine tails,” the shortened form being “the cat.” This whip was used for the physical punishment of sailors who broke certain rules (such as theft, I think). This whip, or “cat” was kept in a bag, perhaps to keep the leather from drying out. Anyway, if a time came for a sailor to receive punishment, “the cat” was taken out of the bag, hence the phrase.
Example: James was planning to surprise his wife with a trip to Japan, a place she’s been wanting to visit. He was supposed to keep this a secret until next week, but he let the cat out of the bag early because he was so excited and wanted her to know.
Credit : Know your phrase
Picture Credit : Google
