What is "stool pigeon"

Stool \Stool\, n. [AS. st[=o]l a seat; akin to OFries. & OS. st[=o]l, D. stoel, G. stuhl, OHG. stuol, Icel. st[=o]ll, Sw. & Dan. stol, Goth. st[=o]ls, Lith. stalas a table, Russ. stol'; from the root of E. stand. [root]163. See Stand, and cf. Fauteuil.]

1. A single seat with three or four legs and without a back, made in various forms for various uses.

2. A seat used in evacuating the bowels; hence, an evacuation; a discharge from the bowels.

3. A stool pigeon, or decoy bird. [U. S.]

4. (Naut.) A small channel on the side of a vessel, for the dead-eyes of the backstays.
   --Totten.

5. A bishop's seat or see; a bishop-stool.
   --J. P. Peters.

6. A bench or form for resting the feet or the knees; a footstool; as, a kneeling stool.

7. Material, such as oyster shells, spread on the sea bottom for oyster spat to adhere to. [Local, U.S.]

   Stool of a window, or Window stool (Arch.), the flat piece upon which the window shuts down, and which corresponds to the sill of a door; in the United States, the narrow shelf fitted on the inside against the actual sill upon which the sash descends. This is called a window seat when broad and low enough to be used as a seat.

   Stool of repentance, the cuttystool. [Scot.]

   Stool pigeon, a pigeon used as a decoy to draw others within a net; hence, a person used as a decoy for others.

"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.

n. (context slang English) A decoy or an informer, especially one who is a spy for the police.

n. a dummy pigeon used to decoy others