Although used in law, this term can be used metaphorically or figuratively. This entails when one is a confused state or state lacking rational thinking. If one is spaced out, one could claim that he or she is non compos mentis.
In English law, the rule of non compos mentis was most commonly used when the defendant invoked religious or magical explanations for behaviour.
Though authorities differ on this idea, the colloquial term nincompoop, meaning a stupid person, may be a distorted form of non compos mentis.