Inability is the lack of power or skill to do something. Not unwillingness — genuine incapacity.
It takes to and a verb, so an inability to concentrate or an inability to pay. The word is formal and careful, and it is used precisely where the distinction from refusal matters, since somebody's inability to pay a fine is treated quite differently from a refusal to pay it. The adjective unable is far commoner in speech, and disability is the related term for a lasting condition.
1nouninabilityThe lack of ability or power to do something.
“His inability to sleep was making everything worse.”
Inability began as a disqualification. The earlier form was inhability, being ruled out of an office, before the word softened into simply not being able.
Proto-Indo-European
*gʰeh₁bʰ-
Latin
habilitās — aptness, ability
Old French
ablete
Middle English
abilite — suitability, aptitude, ability
English
ability
English, today
inability
Built from
From earlier inhability (“disqualification for office”), equivalent to in- + ability. Compare Middle French inhabilité, Medieval Latin inhabilitās. ability First attested in the 1300s. From Middle English abilite (“suitability, aptitude, ability”), from Old French ablete, from Latin habilitās (“aptness, ability”), from habilis (“apt, fit, skillful, able”); equivalent to able + ity.
Etymology from Wiktionary, CC BY-SA