Narc noun – A lawman concerned with narcotics violations. Stool pigeon and narc are semantically related in sneak topic. In some cases you can use "Stool pigeon" instead a noun "Narc", when it comes to topics like informer.
Stool pigeon noun – A person who provides information about another's wrongdoing. Narc and stool pigeon are semantically related in informer to police topic. You can use "Narc" instead a noun phrase "Stool pigeon".