If you're unable to do something, you just can't do it. This isn't necessarily a bad thing! You may be unable to tell a lie, no matter how much you want to blame your brother for breaking that window.

When we're unable, we're incapable. If you're unable to speak French, but you speak German fluently, you might choose to spend the summer in Berlin rather than Quebec City. And when your neighbor is unable to walk her dog during the day, you could volunteer to walk it after school. Being unable is either "lacking ability," or "lacking power."

Definitions of unable

adj (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”
(followed by `of') lacking capacity or ability

adj (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”
(usually followed by `to') having the necessary means or skill or know-how or authority to do something

adj lacking in power or forcefulness

“like an unable phoenix in hot ashes”
ineffective, ineffectual
lacking power or ability

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