WordWeb Online

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

See also: inexcusable, unreasonable, vulnerable