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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]
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!
See stool pigeon