lacking the necessary power, competence, etc., to accomplish some specified act: He was unable to swim.

1350–1400; Middle English; see un-1, able

See incapable.
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World English Dictionary
unable (ʌnˈeɪbəl)
adj (foll by to)
1.  lacking the necessary power, ability, or authority (to do something); not able
2.  archaic incompetent

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
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Word Origin & History

c.1380, from un- (1) "not" + able. Modeled on O.Fr. inhabile or L. inhabilis.
Online Etymology Dictionary, © 2010 Douglas Harper
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Example sentences
Even when both partners seem willing, males are often unable to consummate the
We seem to be bent on destroying every single resource and unable to talk our
  way out of doing so.
Hogan would spend another pain-filled month in the hospital, unable to leave
  his bed.
My family is unable to locate any information on him.
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