WordWeb Online

1. (usually followed by 'to') not having the necessary means or skill or know-how
   "unable to get to town without a car"; "unable to obtain funds"
    
2. (usually followed by 'to') lacking necessary physical or mental ability
   "dyslexics are unable to learn to read adequately"; "the sun was unable to melt enough snow"
    
3. Lacking in power or forcefulness
   "like an unable phoenix in hot ashes"
   - ineffective, ineffectual, effete

See also: ability, impotent, incapable

Antonym: able