The Collaborative International Dictionary of English v.0.48:

Bushwhacker \Bush"whack`er\, n.
   1. One accustomed to beat about, or travel through, bushes.
      [U.S.]
      [1913 Webster]

            They were gallant bushwhackers, and hunters of
            raccoons by moonlight.                --W. Irving.
      [1913 Webster]

   2. A guerrilla; a marauding assassin; one who pretends to be
      a peaceful citizen, but secretly harasses a hostile force
      or its sympathizers. [U.S.] --Farrow.
      [1913 Webster]

WordNet (r) 3.0 (2006):

bushwhacker
    n 1: a disparaging term for an unsophisticated person [syn:
         hillbilly, bushwhacker]
    2: a Confederate guerrilla during the American Civil War

Moby Thesaurus II by Grady Ward, 1.0:

48 Moby Thesaurus words for "bushwhacker":
   Charley, VC, Vietcong, ancestor, announcer, antecedent,
   avant-garde, bellwether, buccinator, bushfighter, casual, explorer,
   forebear, foregoer, forerunner, front runner, frontiersman,
   fugleman, groundbreaker, guerrilla, guide, harbinger, herald,
   innovator, irregular, lead runner, leader, maquis, maquisard,
   messenger, partisan, pathfinder, pioneer, point, precedent,
   precursor, predecessor, resistance, resistance fighter, scout,
   stormy petrel, trailblazer, trailbreaker, underground,
   underground fighter, vanguard, vaunt-courier, voortrekker