Ned Gebhardt, a murder suspect in my latest novel, Something So Divine, is considered feeble-minded by his family and neighbors.
Though common in the 19th century, the definition is frowned upon today, as are other more derogatory terms applied to illnesses or deficiencies of the mind.
Look at censuses from the period and you'll find people labeled as idiots, imbeciles, morons and the like who may have suffered from some degree of mental incapacity or even a physical limitation which hampered normal function.
Ned is not insane, though in 1897, the time of the story, there wasn't even agreement on what constituted sanity, let alone assurance mental capacity could even be considered as a legal defense.
If Ned were arrested and charged with murder today his lawyer would immediately set about arranging a series of psychological tests to determine his competence and probable defense.
Though there are others, Ned is the prime suspect due to his obsession with the victim and a local reputation founded on rumor and gossip. Because the evidence against him is mainly circumstantial, Simon Roth, the investigator, is inclined to give Ned the benefit of the doubt--until he finds damaging evidence.
It is only then Billy McKinney, the lawyer Roth has found for him, decides an insanity plea is the only hope for Ned.
Based on an 1843 British case, the M'Naghten rule had become the standard in both the U.S. and the United Kingdom by which a jury was to decide after hearing testimony by prosecution and defense experts.
In 1972, the American Law Institute developed a new rule for insanity under the Model Penal Code, though some argue that even this is too vague and leaves too much up to the discretion of a jury. About half the states continue to rely on the M'Naghten rule.