The Collaborative International Dictionary of English v.0.48:

Whapping \Whap"ping\, Whopping \Whop"ping\, a.
   Very large; monstrous; astonishing; as, a whapping story.
   [Colloq.]
   [1913 Webster]

WordNet (r) 3.0 (2006):

whopping
    adv 1: extremely; "they all were whopping drunk"
    adj 1: (used informally) very large; "a thumping loss" [syn:
           humongous, banging, thumping, whopping,
           walloping]

Moby Thesaurus II by Grady Ward, 1.0:

37 Moby Thesaurus words for "whopping":
   awful, awfully, banging, big, bumping, clobbering, colossal,
   crushing defeat, damned, decisive defeat, enormous, exceedingly,
   gargantuan, gigantic, highly, hugely, immense, mighty, much,
   no contest, overwhelming defeat, pasting, prodigious, shellacking,
   slapping, smashing defeat, smearing, spanking, thumping,
   thundering, total defeat, utter defeat, walloping, whacking,
   whaling, whitewash, whitewashing