stool pigeon

stool pigeon idiom
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]

The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

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