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.