Source : Webster's Revised Unabridged Dictionary (1913)

Stumper \Stump"er\, n.
   1. One who stumps.

   2. A boastful person. [Slang]

   3. A puzzling or incredible story. [Slang, U.S.]

Source : WordNet®

stumper
     n : a particularly difficult or baffling question or problem
         [syn: {poser}, {toughie}, {sticker}]