stool pigeon
noun [ C ] slang disapproving uk/ˈstuːl ˌpɪdʒ.ən/ us/ˈstuːl ˌpɪdʒ.ən/
（秘密向警方提供情报的）线人，坐探（通常为罪犯）
a person, often a criminal, who gives information in secret to the police so that they can catch other criminals
Compare: fink noun