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