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]

WordNet (r) 3.0 (2006):

trouncing
    n 1: a sound defeat [syn: thrashing, walloping, debacle,
         drubbing, slaughter, trouncing, whipping]
    2: the act of inflicting corporal punishment with repeated blows
       [syn: beating, thrashing, licking, drubbing,
       lacing, trouncing, whacking]

Moby Thesaurus II by Grady Ward, 1.0:

62 Moby Thesaurus words for "trouncing":
   Waterloo, bastinado, basting, battery, beating, belting, buffeting,
   caning, clubbing, collapse, conquering, conquest,
   corporal punishment, cowhiding, crash, cudgeling, deathblow,
   debacle, defeasance, defeat, destruction, downfall, drubbing,
   failure, fall, flagellation, flailing, flogging, fustigation,
   hiding, horsewhipping, lacing, lambasting, lashing, lathering,
   licking, mastery, overcoming, overthrow, overturn, pistol-whipping,
   quietus, rawhiding, rout, ruin, scourging, shellacking, smash,
   spanking, strapping, stripes, subdual, subduing, subjugation,
   swingeing, switching, thrashing, trimming, truncheoning, undoing,
   vanquishment, whipping