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):

defenseless
    adv 1: without defense; "the child was standing in the middle of
           the crossfire, defenselessly" [syn: defenseless,
           defenceless, defenselessly, defencelessly]
    adj 1: lacking protection or support; "a defenseless child"
           [syn: defenseless, defenceless]
    2: having no protecting or concealing cover; "naked to mine
       enemies"- Shakespeare [syn: naked, defenseless]
    3: lacking weapons for self-defense [syn: defenseless,
       defenceless]

Moby Thesaurus II by Grady Ward, 1.0:

38 Moby Thesaurus words for "defenseless":
   abandoned, aidless, bare-handed, deserted, desolate, fatherless,
   forlorn, forsaken, friendless, godforsaken, guardless, guideless,
   helpless, homeless, kithless, leaderless, motherless, outcast,
   pregnable, unarmed, unarmored, unattended, uncovered, undefended,
   unfortified, unfriended, ungarrisoned, unguarded, unprotected,
   unscreened, unsheltered, unshielded, unsuspecting, untenable,
   unwarned, unwatched, vulnerable, weaponless