Narc noun – A lawman concerned with narcotics violations. Rat and narc are semantically related. in sneak topic. In some cases you can use "Rat" instead a noun "Narc", when it comes to topics like informer, informer to police.
Mac, Charlie and Dee had derisive nicknames from the popular kids in high school; Mac was Ronnie the Rat (because he was a "narc"), Charlie was Dirtgrub (because of his poor hygiene), and Dee was the Aluminium Monster (because of her back brace).