The Collaborative International Dictionary of English v.0.48:

Trounce \Trounce\, v. t. [imp. & p. p. Trounced; p. pr. & vb.
   n. Trouncing.] [F. tronce, tronche, a stump, piece of wood.
   See Truncheon.]
   To punish or beat severely; to whip smartly; to flog; to
   castigate. [Colloq.]
   [1913 Webster]