Non compos mentis is a term meaning 'not of sound mind'. Non compos mentis derives from the Latin

Latin

Latin is an Italic language originally spoken in Latium and Ancient Rome. It, along with most European languages, is a descendant of the ancient Proto-Indo-European language. Although it is considered a dead language, a number of scholars and members of the Christian clergy speak it fluently, and...

 non meaning "not", compos meaning "having (command of)", and mentis (genitive singular

Grammatical number

In linguistics, grammatical number is a grammatical category of nouns, pronouns, and adjective and verb agreement that expresses count distinctions ....

 of mens), meaning "mind". It is the direct opposite of Compos mentis (of a composed mind).

Usage

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 not of sound mind.

Also applicable in health care, when a determination of competency needs to be made by a physician for purposes of obtaining informed consent for treatments and, if necessary, assigning a surrogate to make health care decisions. While the proper sphere for this determination is in a court of law, this is practically, and most frequently, made by physicians in the clinical setting.

In English law, the rule of non compos mentis was most commonly used when the defendant invoked religious or magical explanations for behaviour.