WordWeb Online

1. Lacking protection or support
   "a defenceless child"
   - defenseless [US]
    
2. Lacking weapons for self-defence
   - defenseless [US]
    
3. Having no protecting or concealing cover
   "defenceless to mine enemies"
   - naked, defenseless [US]

1. Without defence
   "the child was standing in the middle of the crossfire, defenceless"
   - defenseless [US], defenselessly [US], defencelessly [Brit, Cdn]

See also: defencelessness [Brit, Cdn], unarmed, unprotected, vulnerable