late 13c., "affected with periodic insanity dependent on the changes of the moon," from Old French lunatique"insane," or directly from Late Latin lunaticus "moon-struck," from Latin luna "moon"

Compare Old English monseoc "lunatic," literally "moon-sick;" Middle High German lune "humor, temper, mood, whim, fancy" (German Laune), from Latin luna. Compare also New Testament Greek seleniazomai "be epileptic," from selene "moon." Lunatic fringe (1913) apparently was coined by U.S. politician Theodore Roosevelt.

Then, among the wise and high-minded people who in self-respecting and genuine fashion strive earnestly for peace, there are foolish fanatics always to be found in such a movement and always discrediting it -- the men who form the lunatic fringe in all reform movements. [Theodore Roosevelt, autobiography, 1913].

Earlier it was a term for a type of hairstyle worn over the forehead (1877). Lunatic soup (1933) was Australian slang for "alcoholic drink."

月亮🌙的阴晴圆缺影响地球的潮汐甚至生物周期，古罗马人认为周期性的精神病有时是由月盈月亏引起的。如果个体本来的精神或者肉体状态就不稳定，则更容易受到月亮的侵蚀。

古人相信月亮拥有使人疯狂的魔力，并且这种魔力随着月亮的阴晴圆缺而变化，在满月时魔力最大。如人狼传说中人狼会在满月时变身为狼。

lunatic就是这一认知的产物。它来自拉丁语lunaticus，luna即moon，字面义moonstruck（受到月亮的影响），表示“精神错乱的”。

luna可以进一步溯源到原始印欧语lux，即light。

同时moonstruck本身也是一个词，a moonstruck person会因爱发狂，也会因为失恋而憔悴，但从构词上很容易看出之所以会这样是因为心智受到了月亮的影响。

lunatic asylum，字面义“疯人避难所”，旧时精神病院的婉称，如今使用的时候多带着比喻性的贬义。