stool pigeon

noun3

1. •
   A pigeon tied to a stool by a piece of twine, used as a decoy to entice free-flying birds to the ground to be shot or trapped.
2. •
   (slang) An informer to the authorities, especially one who gives up criminal associates.
3. •
   (slang, no longer used) A decoy used in a crooked gambling game.