stool pigeon 
noun informal
1A police informer.The song is about the ‘victim of someone's evil plan’ who gets turned in by a stool pigeon....
- Willis is on the run, a stool pigeon in flight from Chicago; Perry is in heavy debt, and his calculating, blackmailing wife forces him to squeal to Bruce's vengeful bosses about where the contract killer now lives, for a finder's fee.
- He was a Jew who began his career as a satirist of the Nazis and ended up as their stool pigeon.
2A person acting as a decoy.
Origin
Late 19th century: so named from the original use of a pigeon fixed to a stool as a decoy.