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A "lunatic" is an out-dated term for a person suffering from mental illness.

The term is derived from the latin word for the moon, luna, because some forms of mental illness such as bi-polar disorder and schizophrenia were thought to be directly caused by the phases of the moon.

This notion has long since been dismissed and there is no established relationship between the moon and any form of poor mental health. The term "lunatic" is no longer used anywhere in the medical or mental health professions, but it is still used by tabloid newspapers and in colloquial speech.