Incapacity, n.; pl. Incapacities (.
Etymology: [Cf. F. incapacité.]
1. Want of capacity; lack of physical or intellectual power; inability.
2. (Law)
Want of legal ability or competency to do, give, transmit, or receive something; inability; disqualification; as, the inacapacity of minors to make binding contracts, etc.
Syn. -- Inability; incapability; incompetency; unfitness; disqualification; disability.
Shades of meaning
Fernald has no entry of its own for incapacity; he lists it under idiocy.
Where he writes about it:
Incapacity, or lack of legal qualification for certain acts, necessarily results from imbecility, but may also result from other causes, as from insanity or from age, sex, etc.; as, the incapacity of a minor to make a contract. — from idiocy