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]

Moby Thesaurus II by Grady Ward, 1.0:

150 Moby Thesaurus words for "nonplus":
   Gordian knot, abecedarian, addle, amaze, answer,
   answer conclusively, argue down, astonish, astound, baffle,
   bafflement, balk, bamboozle, beat, bewilderment, blast, boggle,
   bother, brave, bring up short, buffalo, challenge, check,
   checkmate, circumvent, confound, confoundment, confront, confuse,
   confusion, confute, contradict, contravene, controvert, counter,
   counteract, countermand, counterwork, cross, crush, crux, dabbler,
   dash, daze, defeat, defy, demolish, deny, destroy, dilemma,
   dilettante, discomfit, discomposure, disconcert, disconcertedness,
   disconcertion, disconcertment, discountenance, dish, dismay,
   dismiss, dispose of, disrupt, disturbance, dumbfound, elude,
   embarrassment, enigma, faze, finish, fix, floor, flummox, flurry,
   fluster, foil, frustrate, fuddle, get, jam, keep in suspense,
   knock the chocks, knot, knotty point, lick, maze, muddle, mystery,
   mystify, node, nodus, nonprofessional, overcome, overthrow,
   overturn, overwhelm, oxymoron, paradox, paralyze, parry, perplex,
   perplexity, perturbation, pickle, plight, pons asinorum, poser,
   pother, predicament, problem, put to silence, puzzle, puzzlement,
   quandary, rattle, rebut, reduce to silence, refute, riddle, ruin,
   sabotage, scotch, scrape, settle, shock, shut up, silence,
   smash all opposition, smatterer, spike, spoil, squash, squelch,
   stew, stick, stonewall, stop, stump, stun, stymie, subvert,
   take aback, teaser, throw, thwart, tyro, unassuredness, undermine,
   upset, vexed question