The Collaborative International Dictionary of English v.0.48:

Poser \Pos"er\, n.
   One who, or that which, puzzles; a difficult or inexplicable
   question or fact. --Bacon.
   [1913 Webster] Poseur

WordNet (r) 3.0 (2006):

poser
    n 1: a person who habitually pretends to be something he is not
         [syn: poseur, poser]
    2: a person who poses for a photographer or painter or sculptor;
       "the president didn't have time to be a model so the artist
       worked from photos" [syn: model, poser]
    3: a particularly difficult or baffling question or problem
       [syn: poser, stumper, toughie, sticker]

Moby Thesaurus II by Grady Ward, 1.0:

92 Moby Thesaurus words for "poser":
   Chinese puzzle, Gordian knot, Holy Willie, Joseph Surface, Mawworm,
   Pecksniff, Pharisee, Tartuffe, affecter, attitudinarian,
   attitudinizer, baffling problem, blagueur, bluff, bluffer,
   brain twister, canter, canting hypocrite, charlatan,
   crossword puzzle, crux, dilemma, dissembler, dissimulator, enigma,
   enigmatic question, fake, faker, floorer, formalist, fourflusher,
   fraud, humbug, hypocrite, impersonator, impostor, jigsaw puzzle,
   knot, knotty point, lip server, lip worshiper, malingerer,
   mind-boggler, mountebank, mystery, node, nodus, nonplus,
   nut to crack, oxymoron, paradox, perplexed question, perplexity,
   pharisee, phony, pietist, pious fraud, pons asinorum, poseur,
   posture maker, posturer, posturist, pretender, problem, puzzle,
   puzzlement, puzzler, quack, quacksalver, quackster, quandary,
   question, question mark, ranter, religionist, religious hypocrite,
   ringer, saltimbanco, sham, shammer, sixty-four dollar question,
   sniveler, snuffler, spiritual humbug, sticker, striker of poses,
   stumper, teaser, tough proposition, vexed question,
   whited sepulcher, why



The Jargon File (version 4.4.7, 29 Dec 2003):

poser
 n.

    [from French poseur] A wannabee; not hacker slang, but used among
    crackers, phreaks and warez d00dz. Not as negative as lamer or leech.
    Probably derives from a similar usage among punk-rockers and metalheads,
    putting down those who ?talk the talk but don't walk the walk?.