Definition: To reveal a secret carelessly or by mistake.Origin: The original theory of "let the cat out of the bag" is that unscrupulous sellers would sell the "pig" in a bag, but instruct the buyer not to open it until they get home, lest the pig escape. The buyer would carry a wiggling bag all the way home, only to have it revealed when opened that they had received a feral cat instead of a piglet. A whip was also nicknamed to be "the cat" in the Navy. The whip/cat would be taken out of the bag whenever someone had to be disciplined.