Source : Webster's Revised Unabridged Dictionary (1913)

Unable \Un*a"ble\, a.
   Not able; not having sufficient strength, means, knowledge,
   skill, or the like; impotent' weak; helpless; incapable; --
   now usually followed by an infinitive or an adverbial phrase;
   as, unable for work; unable to bear fatigue.

         Sapless age and weak unable limbs.       --Shak.

Source : WordNet®

unable
     adj 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" [syn: {not able}] [ant:
             {able}]
     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"
        [syn: {unable(p)}]
     3: lacking in power or forcefulness; "an ineffectual ruler";
        "like an unable phoenix in hot ashes" [syn: {ineffective},
         {ineffectual}]