Source : Webster's Revised Unabridged Dictionary (1913)

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.

   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.

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

Whap \Whap\, Whop \Whop\, n.
   A blow, or quick, smart stroke.

Source : WordNet®

whap
     v : hit hard [syn: {sock}, {bop}, {whop}, {bonk}, {bash}]
     [also: {whapping}, {whapped}]