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]

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”.