The Collaborative International Dictionary of English v.0.48:

Whap \Whap\, Whop \Whop\, v. t. [imp. & p. p. Whapped; p. pr.
   & vb. n. Whapping.]
   To beat or strike.
   [1913 Webster] Whap

The Collaborative International Dictionary of English v.0.48:

Whap \Whap\, Whop \Whop\, n.
   A blow, or quick, smart stroke.
   [1913 Webster] Whapper

The Collaborative International Dictionary of English v.0.48:

Whap \Whap\, Whop \Whop\, v. i. [Cf. OE. quappen to palpitate,
   E. quob, quaver, wabble, awhape, wap.]
   To throw one's self quickly, or by an abrupt motion; to turn
   suddenly; as, she whapped down on the floor; the fish whapped
   over. --Bartlett.
   [1913 Webster]

   Note: This word is used adverbially in the north of England,
         as in the United States, when anything vanishes, or is
         gone suddenly; as, whap went the cigar out of my mouth.
         [1913 Webster] Whap

WordNet (r) 3.0 (2006):

whap
    v 1: hit hard [syn: sock, bop, whop, whap, bonk,
         bash]