nark
1. noun Someone who acts as an informer or spy for an authority figure, especially law enforcement. The FBI offered to let me avoid jail time if I agreed to be a nark for them. I can't believe you told Mom and Dad! Why do you have to be such a nark?
2. verb To act as an informer or spy for an authority figure. It turned out that she had been narking on the cartel for years. Don't nark on them, Tommy—no one likes a snitch!