The phrase “stool pigeon” comes from the fact that “marks” or victims of con men are called pigeons. In the old days – when operating rigged games such as Monte or Banco (which was so prevalent that the police started to have special squads to deal with them – squads that soon were known as “bunco” squads” because the Irish brogue of many cops confused reporters) – the con man would employ someone who would work for him and who the con man would allow to win. This person sitting on a stool would then help draw others into the game to be fleeced by the con man. When the pair were caught – the cops would press the “stool” “pigeon” to confess and promise a reduced sentence in exchange. Because the guy running the game made most of the money but the stool pigeon would get the same punishment – they often took the deal and provided evidence against the con man running the game. Soon – people forgot the origin of “stool pigeon” and just started to use it to mean anyone who would inform on their friends or associates.