“unable” – English explanatory dictionary

adj not able to do something ○ I regret that I am unable to accept your suggestion. ○ She was unable to come to the meeting. Antonym able (NOTE: be unable to is a rather formal way of saying can’t.)

a. Not able; not having sufficient strength, means, knowledge, skill, or the like; impotent’ weak; helpless; incapable; — now usually followed by an infinitive or an adverbial phrase; as unable for work; unable to bear fatigue. “Sapless age and weak unable limbs.” Shak.