1.Definition: (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"

2.Definition: (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"

3.Definition: lacking in power or forcefulness

"Like an unable phoenix in hot ashes"

Related Adjective(s):ineffective, ineffectual