floorer

from The Collaborative International Dictionary of English v.0.48

Floorer \Floor"er\, n.
   Anything that floors or upsets a person, as a blow that
   knocks him down; a conclusive answer or retort; a task that
   exceeds one's abilities. [Colloq.]
   [1913 Webster]

from Moby Thesaurus II by Grady Ward, 1.0

35 Moby Thesaurus words for "floorer":
      Chinese puzzle, baffling problem, brain twister, clincher,
      conclusive argument, crossword puzzle, crusher, crux, enigma,
      enigmatic question, finisher, jigsaw puzzle, knockdown argument,
      knot, knotty point, mind-boggler, mystery, nut to crack,
      perplexed question, perplexity, poser, problem, puzzle, puzzlement,
      puzzler, question, question mark, settler,
      sixty-four dollar question, sockdolager, sticker, stumper,
      tough proposition, vexed question, why