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