The Collaborative International Dictionary of English v.0.48:

Whack \Whack\, v. t. [imp. & p. p. Whacked; p. pr. & vb. n.
   Whacking.] [Cf. Thwack.]
   1. To strike; to beat; to give a heavy or resounding blow to;
      to thrash; to make with whacks. [Colloq.]
      [1913 Webster]

            Rodsmen were whackingtheir way through willow
            brakes.                               --G. W. Cable.
      [1913 Webster]

   2. To divide into shares; as, to whack the spoils of a
      robbery; -- often with up. [Slang]
      [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

whacked
    adj 1: (British informal) exhausted or worn out