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

The Collaborative International Dictionary of English v.0.48:

Whop \Whop\, v. t.
   Same as Whap. --Forby.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Whop \Whop\, n.
   Same as Whap.
   [1913 Webster]

WordNet (r) 3.0 (2006):

whop
    v 1: hit hard; "The teacher whacked the boy" [syn: whack,
         wham, whop, wallop]
    2: hit hard [syn: sock, bop, whop, whap, bonk, bash]

Moby Thesaurus II by Grady Ward, 1.0:

138 Moby Thesaurus words for "whop":
   bang, bash, baste, bastinado, bat, batter, beat, beating, belabor,
   belt, biff, birch, blank, blow, bonk, bop, buffet, bulldoze, bump,
   burst, bust, cane, chop, clap, clash, clip, clobber, clout, club,
   clump, coldcock, cowhide, crack, crash, cream, crump, cudgel, cut,
   dash, deal, deal a blow, deck, defeat utterly, dig, dint, drub,
   drubbing, drumming, fetch, fetch a blow, flagellate, flail, flap,
   flog, flop, fusillade, fustigate, give a whipping, give the stick,
   hammer, hit, hit a clip, horsewhip, jab, knock, knock cold,
   knock down, knock out, knout, lace, lambaste, lash, lay on,
   let have it, lick, overbear, overwhelm, paste, pelt, pistol-whip,
   plunk, poke, pommel, pound, pummel, punch, rap, rawhide, report,
   schmear, scourge, shellac, shut out, skunk, slam, slap, slat, slog,
   slug, smack, smash, smear, smite, snap, snow under, soak, sock,
   spank, splat, steamroller, strap, strike, strike at, stripe,
   stroke, swap, swat, swing, swinge, swipe, switch, tap, tattoo,
   thrash, thump, thwack, trounce, truncheon, wallop, whack, whale,
   wham, whap, whelm, whip, whitewash, whomp, yerk