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]

The Collaborative International Dictionary of English v.0.48:

Nonplus \Non"plus\, n. [L. non not + plus more, further. See
   Plural.]
   A state or condition which baffles reason or confounds
   judgment; insuperable difficulty; inability to proceed or
   decide; puzzle; quandary.
   [1913 Webster]

         Both of them are a perfect nonplus and baffle to all
         human understanding.                     --South.
   [1913 Webster]

WordNet (r) 3.0 (2006):

nonplus
    v 1: be a mystery or bewildering to; "This beats me!"; "Got me--
         I don't know the answer!"; "a vexing problem"; "This
         question really stuck me" [syn: perplex, vex, stick,
         get, puzzle, mystify, baffle, beat, pose,
         bewilder, flummox, stupefy, nonplus, gravel,
         amaze, dumbfound]