WordWeb Online

1. Incapable of being justified or explained
   - indefensible, insupportable, unjustifiable, unwarrantable
    
2. Without a basis in reason or fact
   "unwarranted jealousy"
   - baseless, groundless, idle, unfounded, wild
    
3. Lacking justification or authorization
   "unwarranted limitations of personal freedom"
   - undue, unjustified

See also: inexcusable, unreasonable, unsupported