This is rarely used in its opposite form, compos mentis, that is, having control of one's faculties. Usually one hears a phrase such as "It was determined that the criminal was non compos mentis" (i.e., insane, not fit to stand trial); one might sometimes say the criminal was compos mentis, i.e., the criminal had full knowledge of his/her actions and is competent to stand trial.
It is typically a legal term, but (in the slang of the United States at least) one often hears phrases such as, "That friend of yours was non compos mentis after getting fired."
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