The Collaborative International Dictionary of English v.0.48:

Whapper \Whap"per\, Whopper \Whop"per\, n. [See Whap.]
   Something uncommonly large of the kind; something
   astonishing; -- applied especially to a bold lie. Now (1998)
   usually spelled whopper. [Colloq.]
   [1913 Webster +PJC] Whapping