\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\, 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.