Definition: An informant; one who discloses secret or vital information.
Example: 1) The police would never have found him if a stool pigeon hadn’t revealed his hiding place.
Etymology: Gangsters in New York started using this phrase in the 1920s. Earlier, in the 1800s, hunters would attach a pigeon (a kind of bird) to a stool (or stand) to act as a decoy, so that other birds would come close and could then be killed. The ‘stool pigeon’ was used to deceive other birds. Among criminals, the ‘stool pigeon’ is the one who is actually working for the police, deceiving the other criminals.