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.

Whacking \Whack"ing\, a.
   Very large; whapping. [Colloq.]

Source : WordNet®

whacking
     adj : (British informal) enormous; "a whacking phone bill"; "a
           whacking lie"
     n : the act of inflicting corporal punishment with repeated
         blows [syn: {beating}, {thrashing}, {licking}, {drubbing},
          {lacing}, {trouncing}]
     adv : extremely; "a whacking good story"