The Collaborative International Dictionary of English v.0.48:

Whacking \Whack"ing\, a.
   Very large; whapping. [Colloq.]
   [1913 Webster]

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):

whacking
    adv 1: extremely; "a whacking good story"
    adj 1: (British informal) enormous; "a whacking phone bill"; "a
           whacking lie"
    n 1: the act of inflicting corporal punishment with repeated
         blows [syn: beating, thrashing, licking, drubbing,
         lacing, trouncing, whacking]

Moby Thesaurus II by Grady Ward, 1.0:

19 Moby Thesaurus words for "whacking":
   awful, banging, bumping, deadly, dreadful, fearful, frightful,
   horrible, howling, rousing, slapping, spanking, terrible, terrific,
   thumping, thundering, walloping, whaling, whopping