The Collaborative International Dictionary of English v.0.48:

Defenseless \De*fense"less\, a.
   Destitute of defense; unprepared to resist attack; unable to
   oppose; unprotected. [Also spelled defenceless.]--
   De*fense"less*ly, adv. -- De*fense"less*ness, n.
   [1913 Webster]

   2. unarmed; used of persons or the military. armed
      [WordNet 1.5]

WordNet (r) 3.0 (2006):

defenselessness
    n 1: the property of being helpless in the face of attack [syn:
         defenselessness, defencelessness, unprotectedness]