stool pigeon

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]