The Latin non compos mentis translates as "of unsound mind": n?n ("not") prefaces compos mentis, meaning "having control of one's mind". This phrase was first used in thirteenth-century English law.
In medieval and early modern Britain, the term Non compos mentis was often related to religious or mysterious phenomena such as diabolical influence. From the seventeenth century, the condition was increasingly viewed as a mental illness, also described as insanity or madness.
In English law, Non compos mentis was a juristic term to describe a person's action as not motivated by reason, but being influenced by some false image or mental impression.Non compos mentis and Felo de se (the Latin word for 'self-murder',) presented two different verdicts in the case of a suicide. In the finding of a jury, the deceased who was stigmatized felo de se, would be excluded from burial in consecrated ground and would forfeit their estate to the Crown, while these penalties would not apply to the deceased affirmed non compos mentis, i.e. deemed to be insane.
Suicide was a severe crime in Tudor and early Stuart England and was considered a form of murder; a sin not only in the eyes of the Church but also defined by criminal law. The state of mind of self-killers at the time they committed their fatal deed was crucial. To be judged guilty of 'self-murder', one had to be sane. Men and women who killed themselves when they were mad or otherwise mentally incompetent were considered innocent. The verdict would be made by a jury. The penalty for suicide in England originated in the ancient world and evolved gradually into their early modern form; similar laws and customs existed in many parts of Europe. Born of domestic beliefs, the ritual of punishing suicide, which is usually concerned with the suicidal corpse, embodies the notion that suicide is polluting, and that the suicide should be ostracized by the community of the living and the dead. The theological and legal severity increased in the High Middle Ages. The medieval theologian Thomas Aquinas extended Augustine's arguments against suicide and added the new interpretation of 'violation of natural law' to it. Most western European governments began to promulgate laws to confiscate some of a suicide's property.
However, attitudes to suicide changed profoundly after 1660, following the English Revolution. After the civil war, political and social changes, judicial and ecclesiastical severity gave way to official leniency for most people who died by suicide. Non compos mentis verdicts increased greatly, and felo de se verdicts became as rare as non compos mentis had been two centuries earlier.
In English law, the rule of non compos mentis was most commonly used when the defendant invoked religious or magical explanations for behaviour, as mentioned above.
Although typically used in law, this term can also be used metaphorically or figuratively; e.g. when one is in a confused state, intoxicated, or similarly mentally impaired. It usually refers to a person has no rational mind or judgement, and whose behaviour is nonsense.
Some relating words such as 'insane', 'mad', and 'lunatic' used more frequently in psychiatry; however, the laws against suicide and the verdicts felo de se and Non compos mentis had never faded until the late nineteenth century.