What does "imbecility" mean?

Imbecility: 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. See Debility.