The Collaborative International Dictionary of English v.0.48:

Nonplus \Non"plus`\, v. t. [imp. & p. p. Nonplusedor
   Nonplussed; p. pr. & vb. n. Nonplusing or Nonplussing.]
   To puzzle; to confound; to perplex; to cause to stop by
   embarrassment.
   [1913 Webster]

         He has been nonplused by Mr. Dry's desiring him to tell
         what it was that he endeavored to prove. --Spectator.
   [1913 Webster]

WordNet (r) 3.0 (2006):

nonplussed
    adj 1: filled with bewilderment; "at a loss to understand those
           remarks"; "puzzled that she left without saying goodbye"
           [syn: at a loss(p), nonplused, nonplussed,
           puzzled]

Moby Thesaurus II by Grady Ward, 1.0:

26 Moby Thesaurus words for "nonplussed":
   addled, at a loss, at a nonplus, at a stand, at a standstill,
   at an impasse, baffled, bamboozled, beat, bewildered, buffaloed,
   confounded, dazed, floored, fuddled, in a dilemma, in suspense,
   licked, muddled, mystified, on tenterhooks, perplexed, puzzled,
   stuck, stumped, thrown