What is Non Compos Mentis?

Legal Definition
The Latin non compos mentis translates as "of unsound mind": nōn ("not") prefaces compos mentis, meaning "having control of one's mind". This phrase was originally used in thirteenth-century British law.
-- Wikipedia
Legal Definition
Persons. These words signify not of sound mind, memory, or understanding. This is a generic term, and includes all the species of madness, whether it arise from, 1, idiocy; 2, sickness 3, lunacy or 4, drunkenness. Co. Litt. 247; 4 Co. 124; 1 Phillim. R. 100; 4 Com. Dig. 613; 5 Com. Dig. 186; Shelf. on Lunatics, 1; and the articles Idiocy; Lunacy.
-- Bouviers Law Dictionary
Legal Definition
Lat Not sound of mind; insane. This is a very general term, embracing all varieties of mental derangement. See Insanity. Coke has enumerated four different classes of persons who are deemed in law to be non compotes mentis: First, an idiot, or fool natural: second, he who was of good and sound mind ana memory, but by the act of God has lost it; third, a lunatic, lunaticus -qui gaudet lucidis intervallis, who sometimes is of good sound mind and memory, and sometimes non compos mentis; fourth, one who is non compos mentis by his own act, as a drunkard. Co.. Litt. 247a; 4 Coke, 124.
-- Black's Law Dictionary
Legal Definition
One wholly bereft of memory and understanding. See 1 Whart. (Pa.) 52, 29 Am. Dec. 33.
-- Ballentine's Law Dictionary