The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

WordNet (r) 3.0 (2006):

unwarrantable
    adj 1: incapable of being justified or explained [syn:
           indefensible, insupportable, unjustifiable,
           unwarrantable, unwarranted]