unanswerable

from WordNet (r) 3.0 (2006)

unanswerable
    adj 1: impossible to answer; "an unanswerable argument"

from The Collaborative International Dictionary of English v.0.48

Unanswerable \Un*an"swer*a*ble\, a.
   Not answerable; irrefutable; conclusive; decisive; as, he
   gave an unanswerable argument. -- {Un*an"swer*a*ble*ness}, n.
   -- {Un*an"swer*a*bly}, adv.
   [1913 Webster]

from Moby Thesaurus II by Grady Ward, 1.0

42 Moby Thesaurus words for "unanswerable":
      absolute, actual, admitting no question, chartered, confirmable,
      demonstrable, demonstratable, excepted, excused, exempt, exempted,
      factual, favored, historical, immune, inappealable, incontestable,
      incontrovertible, indisputable, indubitable, irrefragable,
      irrefutable, irresponsible, let off, licensed, permitted,
      privileged, provable, real, released, self-evident, spared,
      testable, unaccountable, unconfutable, undeniable, unimpeachable,
      unliable, unquestionable, unrefutable, unsubject, verifiable