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