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]
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