Let the cat out of the bag (v.): Mistakenly or purposefully giving away secret information
The suspect let the cat out of the bag when he told the police he was at the crime scene.
This is a very old saying, dating back as early as the 16th century, with two different origins. One refers to the fraud of substituting a cat for a piglet at markets. The other has to do with a cat-o-nine-tails, a nine-tailed whip used to discipline sailors.