Let The Cat Out Of The Bag
To let a secret be known
While letting a cat out of a bag is a kind and humane gesture when you're speaking literally, in the figurative sense the phrase means to let out the truth or, as the above states, to let a secret be known. The phrase has it's origins in the marketplaces of medieval Europe, just like my families history of chronic gout.
Pigs were sold at these markets just like used cars are sold today; customers perused the various offerings and eventually settled on one to purchase. However, imagine if when you were browsing the auto lot all of the cars were driving back and forth, whipping themselves into a frenzy. That's what pigs do when you try to confine them to small spaces, and the market stalls back then were very small indeed. Therefore, in that beautiful age before PETA when cruelty reigned free, livestock dealers simply put the pigs into burlap sacks to keep them somewhat docile.
But where does this cat business come into it? Well, just like the kid sold me an ounce of oregano in middle school, there were hustlers back then as well. These frauds would put a cat in the burlap sack instead of a piglet and, since no one wanted to risk chasing a pig around the crowded marketplace, the dirty deed often wasn't discovered till the buyer arrived back at his cave or whatever they lived in back then. Once the bag was opened, the secret was out and the cat was, literally, out of the bag.
An angry buyer vents his frustrations at a fraudulent purchase