nark

1. (UK, Ireland) A police spy or informer.Examples: "So I went and laid down on the grass. While laying there I piped a reeler whom I knew. He had a nark (a policeman's spy) with him. So I went and looked about for my two pals, and told them to look out for F. and his nark."; "the bystander. […] She thought you was a copper's nark, sir. / the note taker [with quick interest] Whats a copper's nark? / the bystander [inapt at definition] It's a—well, it's a copper's nark, as you might say. What else would you call it? A sort of informer."; "When we got to the Chief of Police's office a crowd of the most dreadful-looking scoundrels, obviously police narks, informers, and spies of every kind, were hanging about outside the door."Synonyms: abaddon, accuser, beefer, bewrayer, bigmouth, blabber, blabtale, blabtongueIrelandUKslang

2. (Australia) An unpleasant person, especially one who makes things difficult for others.Synonyms: spoilsport, assbucket, asshat, asshole, assmonkey, assmouth, assmunch, assmuncherAustraliaslang