Stool pigeon

This expression first made its appearance in America meaning a police informer in the early 19th century. It is thought to derive from the horrendous hunting practice of tying a live, blinded pigeon to a ‘stool’ or tree stump, to act as a decoy to attract other pigeons. From the early 20th century, the meaning expanded from police informer to a criminal who betrayed his accomplices in return for leniency.