The Collaborative International Dictionary of English v.0.48:

Thwack \Thwack\ (thw[a^]k), v. t. [imp. & p. p. Thwacked; p.
   pr. & vb. n. Thwacking.] [Cf. OE. thakken to stroke, AS.
   [thorn]accian, E. whack.]
   1. To strike with something flat or heavy; to bang, or
      thrash: to thump. "A distant thwacking sound." --W.
      Irving.
      [1913 Webster]

   2. To fill to overflow. [Obs.] --Stanyhurst.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Thwack \Thwack\, n.
   A heavy blow with something flat or heavy; a thump.
   [1913 Webster]

         With many a stiff thwack, many a bang,
         Hard crab tree and old iron rang.        --Hudibras.
   [1913 Webster]

WordNet (r) 3.0 (2006):

thwack
    n 1: a hard blow with a flat object
    v 1: deliver a hard blow to; "The teacher smacked the student
         who had misbehaved" [syn: smack, thwack]

Moby Thesaurus II by Grady Ward, 1.0:

85 Moby Thesaurus words for "thwack":
   bang, bash, bat, beating, belt, biff, blow, bonk, bop, bump, burst,
   bust, chop, clap, clash, clip, clobber, clout, clump, coldcock,
   crack, crash, crump, cut, dash, deal, deal a blow, deck, dig, dint,
   drub, drubbing, drumming, fetch, fetch a blow, flap, flop,
   fusillade, hit, hit a clip, jab, knock, knock cold, knock down,
   knock out, let have it, lick, paste, pelt, plunk, poke, pound,
   punch, rap, report, slam, slap, slat, slog, slug, smack, smash,
   smite, snap, soak, sock, splat, strike, strike at, stroke, swap,
   swat, swing, swipe, tap, tattoo, thump, wallop, welt, whack, wham,
   whap, whomp, whop, yerk