poser (n.1)

"one who practices an affected attitude," 1881, agent noun from pose (v.1); revived in teenager slang by 1983.

poser (n.2)

"question that puzzles," 1793 from pose (v.2) or from apposer; earlier it meant "one who asks testing questions" (1580s). 

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Definitions of poser from WordNet

poser (n.)
a person who habitually pretends to be something he is not;
Synonyms: poseur
poser (n.)
a person who poses for a photographer or painter or sculptor;
Synonyms: model
poser (n.)
a particularly difficult or baffling question or problem;
Synonyms: stumper / toughie / sticker