unable

Unable is an adjective meaning lacking the power or means to do something; not able. It denotes a state of constraint or lack of ability, which can be temporary or persistent. In contemporary English, unable is typically followed by a to-infinitive to specify the action that cannot be performed, as in "She was unable to attend the meeting."

Grammar and usage notes define unable as a predicative adjective that appears after linking verbs such as is, was, were, or have been. It normally takes an infinitive clause: "unable to finish," "unable to reach the destination." In formal writing, able and not able are often avoided in favor of unable and the to-infinitive.

Etymology and related terms: unable is formed from the prefix un- (not) plus able (capable). The word able derives from Latin habilis via Old French habile. The construction dates to the Middle English period. Related terms include ability, incapability or inability, able, and incapable, which express different nuances of capability.

Usage notes: unable can describe physical, cognitive, or logistical constraints, as in "The device is unable to connect to the network" or "They were unable to provide an answer." It is commonly found in formal writing, professional communications, and journalism. Its common antonyms are able or capable, while the phrase not able to is more informal and often avoided in formal style.