Non compos mentis
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The term non compos mentis comes from Latin, non meaning "not," compos meaning "in control," and mentis, genitive singular of mens, mind. and means not having a sound mind; not sane.
It is most typically used in its negative form, non compos mentis, that is, not having control of one's faculties, as in a phrase such as "It was determined that the criminal was non compos mentis" (i.e., insane); one might say the criminal was compos mentis, i.e., the criminal had full knowledge of his/her acts and is competent to stand trial.
Although it is used as a legal term, it can also be used colloquially. In its colloquial form the expression is often less severe than the strict legal sense. For example, "He was not fully compos mentis as he was still suffering from a hangover" means he was not fully in charge of his faculties. Another example, using the negative form, is "I'm feeling a bit non compos mentis today," meaning "I'm feeling a bit out of it" or "...a bit spaced out".
The colloquial word nincompoop = "stupid person" may be (authorities differ on this) a distorted form of non compos mentis.