1. A Narcotics officer.
2. A tattle-tale, someone who would tell parents/boss/police/authority figure/the man about a misdeed.
3. Someone who hates care-free fun, usually a similar type of person to 2. Will often rain on your parade with facts about the environment when you get a new car. Will complain about practical jokes being mean, or play devil's advocate when it is unecessary or nearly impossible.
4. A person who is short tempered or irratable. Will often shout at you for being on their lawn.
I think that guy might be a Narc, you should hide your stash.
God, Phil, you're such a narc. I can't believe you told the teacher I threw the paper.
Stop being such a fucking narc, Sim. You can't outrun a bear and child abuse IS wrong.
Calm down, you've turned into such a narc lately.