The Collaborative International Dictionary of English v.0.48:

Indefensible \In`de*fen"si*ble\, [Pref. in- not + defensible:
   cf. OF. indefensible, indefensable.]
   Not defensible; not capable of being defended, maintained,
   vindicated, or justified; unjustifiable; untenable; as, an
   indefensible fortress, position, cause, statement, etc.
   [1913 Webster]

         Men find that something can be said in favor of what,
         on the very proposal, they thought utterly
         indefensible.                            --Burke.
   [1913 Webster]

WordNet (r) 3.0 (2006):

indefensible
    adj 1: (of theories etc) incapable of being defended or
           justified [syn: indefensible, untenable]
    2: not able to be protected against attack
    3: incapable of being justified or explained [syn:
       indefensible, insupportable, unjustifiable,
       unwarrantable, unwarranted]

Moby Thesaurus II by Grady Ward, 1.0:

19 Moby Thesaurus words for "indefensible":
   exceptionable, impossible, inadmissible, inexcusable, inexpiable,
   intolerable, irremissible, objectionable, unacceptable,
   unallowable, unconscionable, undesirable, unforgivable,
   unjustifiable, unpardonable, unreasonable, unsuitable, untenable,
   unwarrantable