The Collaborative International Dictionary of English v.0.48:

Untenable \Untenable\
   See tenable.

WordNet (r) 3.0 (2006):

untenable
    adj 1: (of theories etc) incapable of being defended or
           justified [syn: indefensible, untenable]

Moby Thesaurus II by Grady Ward, 1.0:

51 Moby Thesaurus words for "untenable":
   aidless, baseless, defenseless, empty, exceptionable, fatherless,
   faulty, flawed, friendless, groundless, guideless, helpless, idle,
   ill-founded, illogical, implausible, impossible, inadmissible,
   indefensible, inexpiable, insupportable, intolerable, leaderless,
   motherless, objectionable, pregnable, specious, unacceptable,
   unbased, undesirable, unforgivable, unfounded, unfriended,
   ungrounded, unjustifiable, unjustified, unpardonable, unprotected,
   unreasonable, unsound, unsuitable, unsupportable, unsupported,
   unsustainable, unsustained, unwarranted, vain, vulnerable, weak,
   without basis, without foundation