Source : Webster's Revised Unabridged Dictionary (1913)

Defenseless \De*fense"less\, a.
   Destitute of defense; unprepared to resist attack; unable to
   oppose; unprotected. -- {De*fense"less*ly}, adv. --
   {De*fense"less*ness}, n.

Source : WordNet®

defenselessness
     n : the property of being helpless in the face of attack [syn: {defencelessness},
          {unprotectedness}]