Let the cat out of the bag

Two competing legends explain this one, and neither has solid proof behind it. The market-fraud theory says dishonest traders would sell a sack supposedly containing a piglet, but secretly swap in a worthless cat, with the fraud exposed the moment the bag was opened (this theory also tries to explain "a pig in a poke"). The naval theory points to the cat o' nine tails whip being kept in a cloth bag aboard ship, with "letting the cat out of the bag" meaning a punishment was about to be revealed. The earliest recorded use, from 1760, is a book review complaining that the ending had been given away, already a fairly abstract, secret-revealing sense, with no mention of literal cats, pigs, or whips.