stool pigeon (n.)

"police informer," 1859, American English; earlier "one who betrays the unwary (or is used to betray them)," 1821, originally a decoy bird (1812); said to be from decoys being fastened to stools to lure other pigeons. But perhaps related to stall "decoy bird" (c. 1500), especially "a pigeon used to entice a hawk into the net" (see stall (n.2)). Also see pigeon.