WordNet (r) 3.0 (2006):

flummox
    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:

104 Moby Thesaurus words for "flummox":
   abash, addle, addle the wits, baffle, balk, ball up, becloud,
   bedazzle, befuddle, bewilder, blast, bomb, bother, brave, bug,
   challenge, checkmate, circumvent, cloud, confound, confront,
   confuse, contravene, counter, counteract, countermand, counterwork,
   crap out, cross, dash, daze, dazzle, deceive, defeat, defy,
   destroy, discombobulate, discomfit, discompose, disconcert,
   discountenance, dish, dismay, disorganize, disorient, disrupt,
   disturb, draw a blank, drop a bomb, drop the ball, elude,
   embarrass, entangle, fan, flop, flurry, fluster, flutter, fog,
   foil, fold, fold up, fox, frustrate, fuddle, fuss, get left,
   hoodwink, knock the chocks, lay an egg, lose out, maze, mist,
   mix up, moider, muddle, mystify, nonplus, not hack it, not make it,
   perplex, perturb, pother, put out, puzzle, raise hell, rattle,
   ruffle, ruin, sabotage, scotch, spike, spoil, stonewall,
   strike out, stump, stymie, take the count, throw into confusion,
   thwart, unsettle, upset, wash out, whiff