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 informer(向警察提供情报的)线人,内线 词源late 19th cent.: so named from the original use of a pigeon fixed to a stool as a decoy.