The Collaborative International Dictionary of English v.0.48:

Poacher \Poach"er\ (p[=o]ch"[~e]r), n.
   1. One who poaches; one who kills or catches game or fish
      contrary to law.
      [1913 Webster]

   2. (Zool.) The American widgeon. [Local, U.S.]
      [1913 Webster]

   Sea poacher (Zool.), the lyrie.
      [1913 Webster]

WordNet (r) 3.0 (2006):

poacher
    n 1: someone who hunts or fishes illegally on the property of
         another
    2: a cooking vessel designed to poach food (such as fish or
       eggs)
    3: small slender fish (to 8 inches) with body covered by bony
       plates; chiefly of deeper northern Pacific waters [syn:
       poacher, sea poacher, sea poker]

Moby Thesaurus II by Grady Ward, 1.0:

32 Moby Thesaurus words for "poacher":
   body snatcher, booster, chicken thief, con man, crook,
   den of thieves, embezzler, filcher, ganef, ghoul, grafter,
   grave robber, jewel thief, land pirate, land shark, land-grabber,
   larcener, larcenist, lifter, peculator, petty thief, pilferer,
   prowler, purloiner, robber, scrounger, shoplifter, sneak thief,
   stealer, swindler, thief, white-collar thief