The Collaborative International Dictionary of English v.0.48:
Stumper \Stump"er\, n. 1. One who stumps. [1913 Webster] 2. A boastful person. [Slang] [1913 Webster] 3. A puzzling or incredible story. [Slang, U.S.] [1913 Webster]WordNet (r) 3.0 (2006):
stumper n 1: a particularly difficult or baffling question or problem [syn: poser, stumper, toughie, sticker]