Non compos mentis is a borrowed phrase or loan phrase. We will look at the definition of the term bon vivant, where it came from, and some examples of its use in sentences.
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.
Examples
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)