To render unable; disable
"The sudden ice storm left everyone unable to drive home safely."
"I am unable to open the jar because the lid is stuck."
(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') lacking necessary physical or mental ability
"dyslexics are unable to learn to read adequately"
"the sun was unable to melt enough snow"
lacking in power or forcefulness
"an ineffectual ruler"
"like an unable phoenix in hot ashes"
Not able; lacking a certain ability.
"After breaking his leg, he was unable to run even for short distances."
In plain English: Unable means not having the ability to do something.
"He was unable to finish the race due to his injury."
Usage: Use unable to describe someone who lacks the capacity or opportunity to do something, often followed by an infinitive verb like "to finish." It is more specific than simply saying "not," as it implies that success was blocked by skill, resources, or circumstances rather than just a choice.
The word unable comes from Middle English as a combination of the prefix un- and able. It has retained its original meaning since entering the language centuries ago.