Imbecility, n.; pl. Imbecilities.
Etymology: [L. imbecillitas: cf. F. imbécillité.]
The quality of being imbecile; weakness; feebleness, esp. of mind. Cruelty . . . argues not only a depravedness of nature, but also a meanness of courage and imbecility of mind. Sir W. Temple.
Note: This term is used specifically to denote natural weakness of the mental faculties, affecting one's power to act reasonably or intelligently.
Syn. -- Debility; infirmity; weakness; feebleness; impotence. See Debility.
Shades of meaning
Fernald has no entry of its own for imbecility; he lists it under idiocy.
Where he writes about it:
Imbecility is a condition of mental weakness, which may or may not be as complete as that of idiocy, but is at least such as to incapacitate for the serious duties of life. — from idiocy
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
Idiocy or imbecility is weakness of mind, while insanity is disorder or abnormal action of mind. — from idiocy
Stupidity is dulness and slowness of mental action which may range all the way from lack of normal readiness to absolute imbecility. — from idiocy