stool ‧ pi ‧ geon /ˈstuːlˌpɪdʒən, ˈstuːlˌpɪdʒɪn/ BrE AmE noun [countable] American English informal
[ Date: 1800-1900 ; Origin: stoolpigeon 'pigeon tied to a wooden frame to attract other birds for shooting at' (1800-1900) ]
someone, especially a criminal, who helps the police to catch another criminal, usually by giving them information SYN informer