supergrass
noun [ C ] UK uk/ˈsuː.pə.ɡrɑːs/ us/ˈsuː.pɚ.ɡræs/
向警方告密者，举报人（尤指揭发其他重案犯的罪犯）
a person, especially a criminal, who gives the police a lot of information about the activities of criminals, especially serious ones
Compare: fink noun sneak noun (PERSON) snitch noun stool pigeon telltale noun tattletale
See also: grass noun