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An informant, especially a criminal working as a spy for law enforcement officers. Although the criminal agreed to help the police in order to avoid prison time, he was afraid the other gang members would kill him if they discovered he was a stool pigeon.
A decoy or informer, especially a police spy. For example, Watch out for Doug; I'm sure he's a stool pigeon for the supervisor. This term alludes to a bird tied to a stool or similar perch in order to attract other birds, which will then be shot. However, one writer believes that stool is a variant for stale or stall, both nouns used for a decoy bird before 1500 or so. [c. 1820]
a stool pigeonOLD-FASHIONED
A stool pigeon is someone who gives secret information to the police. There's nothing the mob hates more than a stool pigeon. Note: This expression is used to express disapproval. Note: This expression comes from the old practice of putting a wooden pigeon on a seat to trick other pigeons.
stool (pigeon)and stoolie (ˈstul ˈpɪdʒən and ˈstuli)
n. an informer. (Originally underworld.) Some stool pigeon spilled the works to the boys in blue. I’m no stoolie!