A nark is British slang for an informer - a shortening of copper's nark

Now this is interesting, zootsuit! As Dr. Bill said, here in the US narc means, likewise, an informer or undercover cop, but derived from narcotics and the drug culture of the 60's (maybe earlier?) Back in the 60's and 70's the worst thing you could call somebody or accuse them of being was a narc! What are you, a narc?" is still a phrase you hear fairly regulary, usually in a jocular sense. But I've never seen it spelled with a "k", nor was I ever aware it was also derived from another source across the pond. So I'd love to hear some more about the background/etymology of copper's nark, and nark with the "k" as well.