"A decoy bird, or a police informer, or criminal's look-out or decoy."
The origin seems plausible and interesting:
"Most reference sources say that this expression derives from the hunting practice of fixing a dead or replica pigeon on to a stool to act as a decoy to attract other birds.
What the stool in question was isn't entirely clear. It certainly wasn't the three-legged piece of furniture we now know, but one of the many other meanings of the word. In the 16th century a 'stoale' was the base of a tree - what we would now call a stump, just the place for a decoy bird to sit. It is also possible that 'stool' is derived from 'estale', which is an early French word applied to a pigeon used to entice a hawk into a net. It isn't far from 'estale pigeon' to 'stool pigeon'.
All of that seems quite straightforward, except for the fact that the term 'stool pigeon', or 'stoolie', doesn't appear in print until the 19th century and in a completely different context. It is first used in American publications and referred to criminals who lured others into crime rather than to decoy birds."
Beware the stool pigeon!