1. Incapable of being justified or explained
   "Such sanctions are utterly indefensible";
   - insupportable, unjustifiable, unwarrantable, unwarranted
    
2. (of theories etc.) incapable of being defended or justified
   "One can put oneself into a false position, ultimately into an indefensible position";
   - untenable
    
3. Not able to be protected against attack
   "His indefensible position in the debate left him vulnerable to criticism"

See also: inexcusable, unreasonable, vulnerable

Encyclopedia: Indefensible