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]

WordNet (r) 3.0 (2006):

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

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