The Collaborative International Dictionary of English v.0.48:

defenceless \defenceless\ adj.
   same as defenseless; as, a defenceless child.

   Syn: defenseless, helpless.
        [WordNet 1.5]

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):

defenceless
    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: lacking weapons for self-defense [syn: defenseless,
       defenceless]

The Devil's Dictionary (1881-1906):

DEFENCELESS, adj.  Unable to attack.