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):

nonplused
    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]