Non compos mentis is a borrowed phrase or loan phrase. Borrowed phrases or loan phrases are terms that have been taken from other languages and used as English phrases. Borrowed phrases or loan phrases enter the English language when English speakers come into contact with other languages and cultures. When borrowed phrases or loan phrases first enter the English language, they are used by bilingual speakers and usually maintain the original pronunciation from the source language. As other English speakers adopt them, the pronunciation may change to incorporate sounds more in keeping with the English speakers’ accents. Do not confuse borrowed phrases or loan phrases with calques. A calque is a loaned translation; it is a word or phrase which adopts the meaning of a foreign word or phrase with existing English words. Some examples of calques are the English word bushmeat taken from the French word viande de brousse, and the English phrase rest in peace derived from the Latin phrase requiescat in pace. English has incorporated borrowed phrases or loan phrases from many foreign languages. For instance, the terms modus operandi and quid pro quo are borrowed from Latin, à la carte and gaffe are borrowed from French, karaoke and bokeh are borrowed from Japanese, and loot and nirvana are borrowed from Hindi. English is an ever-evolving language that is somewhat of a melting pot of other languages and cultures.
Non compos mentis means mentally incompetent, insane, not in one’s right mind. The term is often used when mounting an insanity defense that will determine whether a defendant is found not guilty due to insanity in a criminal trial, in a hearing that will determine whether the defendant is competent to stand trial at all, or in an insanity hearing that will determine whether someone mentally ill will be committed to a psychiatric hospital, either a private or state hospital. Non compos mentis is primarily a legal term, but it may also be found in literature. The phrase non compos mentis is Latin and translates as “of unsound mind.” The phrase non compos mentis legally describes someone who has been driven insane by accident, medical issues, or circumstances; it does not describe someone mentally incompetent since birth. The phrase non compos mentis has been used in English at least since the 1600s.
As the host began explaining to the audience that ‘Thomas Bethune’ was an idiot, a blind, almost savage creature, one that had been officially determined non compos mentis by a medical doctor. (Knox Pages)
Eleven states still have laws banning “insane persons” or those who are “non compos mentis” from voting. (Dallas Morning News)
In effect, biology has so impaired their judgment and decision-making that they might as well be considered non compos mentis — legally insane — as far as the courts are concerned. (Baltimore Sun)