Source : Webster's Revised Unabridged Dictionary (1913)

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.

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

Thwack \Thwack\, n.
   A heavy blow with something flat or heavy; a thump.

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

Source : WordNet®

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