“Lunatic” is an informal term referring to people who are considered mentally ill, dangerous, foolish or unpredictable; conditions once called lunacy. The term may be considered insulting in serious contexts, though is now more likely to be used in friendly jest. The word derives from lunaticus meaning “of the moon” or “moonstruck.” By the fourth and fifth centuries astrologers began to commonly use the term to refer to neurological and psychiatric diseases.
Philosophers such as Aristotle and Pliny the Elder argued that the full Moon induced insane individuals with bipolar disorder by providing light during nights which would otherwise have been dark, and affecting susceptible individuals through the well-known route of sleep deprivation. Through at least 1700 it was also a common belief that the Moon influenced fevers, rheumatism, episodes of epilepsy and other diseases.
In the British jurisdiction of England and Wales the Lunacy Acts 1890-1922 referred to lunatics, but the Mental Treatment Act 1930 changed the legal term to “Person of Unsound Mind,” an expression which was replaced under the Mental Health Act 1959 by “mental illness.” “Person of Unsound Mind” was the term used in 1950 in the English version of the European Convention on Human Rights as one of the types of person who could be deprived of liberty by a judicial process. The 1930 act also replaced “asylum” with “mental hospital.” Criminal lunatics became Broadmoor patients in 1948 under the NHS (Scotland) Act 1947.
“Of Unsound Mind” or non compos mentis are alternatives to “lunatic,” which was the most conspicuous term used for insanity in the law in the late 19th century.
Taken from the Wikipedia entry for Lunatic. Because clearly, since I’m no longer a student, I have forgotten how to conduct academic research and have consequently run into the arms of darling Wiki, with its questionable statements and suspicious sources.