Lunatic: The belief that the full moon… | etymologist.ai
lunatic
/ˈluː.nə.tɪk/·noun·c. 1290·Established
Origin
From Latin 'lunaticus' (moonstruck) — preserving the ancient belief that the full moon causesmadness.
Definition
A person who is mentally ill (archaic/offensive); informally, a wildly foolish person.
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Old French / Late Latin13th centurywell-attested
From OldFrench 'lunatique' (insane, moon-struck), from Late Latin 'lunaticus' (moon-struck, of unsound mind), derived from Latin 'luna' (moon). The belief that the moon caused or exacerbated mental illness was widespread in antiquity and the medieval period; the full moon was held to trigger episodes of madness, a view embedded in folklore from Rome to Arabia. Latin 'luna' comes from PIE
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The belief that the full moon causesmadness is ancient and widespread — and still alive. The word 'lunatic' preserves it permanently. Even today, police officers, hospital workers, and teachers report that full-moon nights are crazier than average, though controlled studies have never confirmed a 'lunar effect.' The PIE root *lewk- (light) connects 'lunatic' to an enormous family: 'light' (through Germanic), 'lucid' (clear as light), 'illuminate
of 1959. The informal colloquial sense (wildly foolish person) is now the dominant one. Key roots: lūna (Latin: "moon"), *lewk- (Proto-Indo-European: "light, brightness").