Source : Webster's Revised Unabridged Dictionary (1913)

Unwarrantable \Un*war"rant*a*ble\, a.
   Not warrantable; indefensible; not vindicable; not
   justifiable; illegal; unjust; improper. --
   {Un*war"rant*a*ble*ness}, n. -- {Un*war"rant*a*bly}, adv.

Source : WordNet®

unwarrantable
     adj : incapable of being justified or explained [syn: {indefensible},
            {insupportable}, {unjustifiable}, {unwarranted}]