Imbecility
Imbecility , noun
[Latin imbecillitas: compare French 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.
This term is used specifically to denote natural weakness of the mental faculties, affecting one's power to act reasonably or intelligently.