Source : Webster's Revised Unabridged Dictionary (1913)

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, etc.

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

Source : WordNet®

indefensible
     adj 1: (of theories etc) incapable of being defended or justified
            [syn: {untenable}]
     2: incapable of being justified or explained [syn: {insupportable},
         {unjustifiable}, {unwarrantable}, {unwarranted}]