Source : Webster's Revised Unabridged Dictionary (1913)

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

         Rodsmen were whackingtheir way through willow brakes.
                                                  --G. W. Cable.

Whack \Whack\, v. i.
   To strike anything with a smart blow.

   {To whack away}, to continue striking heavy blows; as, to
      whack away at a log. [Colloq.]

Whack \Whack\, n.
   A smart resounding blow. [Colloq.]

Whack \Whack\, v. t.
   To divide into shares; as, to whack the spoils of a robbery;
   -- often with up. [Slang]

Whack \Whack\, n.
   A portion; share; allowance. [Slang]

   {Out of whack}, out of order. [Slang]

Source : WordNet®

whack
     n 1: the sound made by a sharp swift blow
     2: the act of hitting vigorously; "he gave the table a whack"
        [syn: {knock}, {belt}, {rap}, {whang}]

whack
     v : hit hard; "The teacher whacked the boy" [syn: {wham}, {whop},
          {wallop}]

Source : Free On-Line Dictionary of Computing

whack
     
        According to arch-hacker James Gosling, to "...modify a
        program with no idea whatsoever how it works." (See
        {whacker}.)  It is actually possible to do this in nontrivial
        circumstances if the change is small and well-defined and you
        are very good at {glark}ing things from context.  As a trivial
        example, it is relatively easy to change all "stderr" writes
        to "stdout" writes in a piece of C filter code which remains
        otherwise mysterious.
     
        [{Jargon File}]