The Collaborative International Dictionary of English v.0.48:

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.
      [1913 Webster]

   2. A seat used in evacuating the bowels; hence, an
      evacuation; a discharge from the bowels.
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   3. A stool pigeon, or decoy bird. [U. S.]
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   4. (Naut.) A small channel on the side of a vessel, for the
      dead-eyes of the backstays. --Totten.
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   5. A bishop's seat or see; a bishop-stool. --J. P. Peters.
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   6. A bench or form for resting the feet or the knees; a
      footstool; as, a kneeling stool.
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   7. Material, such as oyster shells, spread on the sea bottom
      for oyster spat to adhere to. [Local, U.S.]
      [1913 Webster]

   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.
      [1913 Webster]

WordNet (r) 3.0 (2006):

stool pigeon
    n 1: someone acting as an informer or decoy for the police [syn:
         fink, snitch, snitcher, stoolpigeon, stool
         pigeon, stoolie, sneak, sneaker, canary]
    2: a dummy pigeon used to decoy others

Moby Thesaurus II by Grady Ward, 1.0:

29 Moby Thesaurus words for "stool pigeon":
   agent provocateur, betrayer, blab, blabber, blabberer,
   blabbermouth, canary, capper, come-on man, decoy, delator, fink,
   informer, narc, nark, peacher, plant, shill, snitch, snitcher, spy,
   squealer, stoolie, talebearer, tattler, tattletale, telltale,
   tipster, whistle-blower