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]