stool pigeon
noun/ˈstuːl pɪdʒɪn/
/ˈstuːl pɪdʒɪn/
(informal)- a person, especially a criminal, who helps the police to catch another criminal, for example by spending time with them and getting secret information synonym informerWord Originlate 19th cent.: so named from the original use of a pigeon fixed to a stool as a decoy.
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stool pigeon