Noun
(originally) A pigeon tied to a stool by a piece of twine, used as a decoy to entice free-flying birds to the ground to be shot or trapped.
(slang, obsolete) A decoy used in a crooked gambling game.
(slang) An informer to the authorities, especially one who gives up criminal associates.
Synonyms: stoolie, stooly; see also Thesaurus:informant
stool-pigeon