- noun an informer. In North America in the 19th century pigeons were tied to wooden frames (known as stools) as decoys to lure game birds. The expression was later applied to a cardsharp’s human decoy, and later still to a police informer or spy. By World War I the use of the phrase had spread to Britain where it was adopted by crime fiction and the real underworld. The term is commonly shortened to stoolie.