| 1. | a pigeon used as a decoy. |
| 2. | Also called stool⋅ie [stoo-lee] , stooly. Slang. a person employed or acting as a decoy or informer, esp. for the police. |

| stool pigeon n.
[From the practice of tying decoy pigeons to a stool to attract other pigeons.] |
An informer, especially for the police: “Lefty figured out that Mugsy was the stool pigeon when he saw him talking to the warden.”
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stool (pigeon)
[ˈstul ˈpɪdʒən] and [ˈstuli] and stoolie
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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]