The Collaborative International Dictionary of English v.0.48:

Bushwhacking \Bush"whack`ing\, n.
   1. Traveling, or working a way, through bushes; pulling by
      the bushes, as in hauling a boat along the bushy margin of
      a stream. [U.S.] --T. Flint.
      [1913 Webster]

   2. The crimes or warfare of bushwhackers. [U.S.]
      [1913 Webster]

WordNet (r) 3.0 (2006):

bushwhacking
    adj 1: lying in ambush; "bushwhacking guerrillas attacking from
           ambush"