Etymology
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lunatic (adj.)

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" (see luna).

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 selēniazomai "be epileptic," from selēnē "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 (1918) was slang for "alcoholic drink" or in reference to drinking several different alcoholic drinks together.

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lunatic (n.)
"lunatic person," late 14c., from lunatic (adj.). Originally one with lucid intervals; later, in legal use, a general term for a person of unsound mind.
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lunacy (n.)
1540s, "condition of being a lunatic," formed irregularly in English from lunatic (q.v.) + -cy. Originally in reference to intermittent periods of insanity, such as were believed to be triggered by the moon's cycle. The Old English equivalent was monaðseocnes "month-sickness." In later legal use, any unsoundness of mind sufficient to render one incapable of civil transactions or management of one's affairs. Weakened figurative sense "act of madness or folly" is from 1580s.
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loony (adj.)

also loonie, looney, luny, "crazy; silly and eccentric," 1853, American English, short for lunatic, but also influenced by loon (n.2) and perhaps loon (n.1), the bird being noted for its wild cry and method of escaping from danger. As a noun by 1884, from the adjective.

Slang loony bin "insane asylum" is by 1909. Looney left in reference to holders of political views felt to be left-wing in the extreme is from 1977. Looney Tunes, Warner Bros. studios' animated cartoon series, dates from 1930.

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insane (adj.)
1550s, of persons, "mentally damaged," from Latin insanus "mad, insane, of unsound mind; outrageous, excessive, extravagant," from in- "not" (see in- (1)) + sanus "well, healthy, sane" (see sane). In reference to actions, "irrational, evidencing madness," from 1842 in English. The noun meaning "insane person" is attested from 1786. For the notion of insanity as sickness, compare lunatic; and Italian pazzo "insane," originally a euphemism, from Latin patiens "suffering." German verrückt, literally past participle of verrücken "to displace," "applied to the brain as to a clock that is 'out of order' " [Buck].
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*leuk- 

Proto-Indo-European root meaning "light, brightness."

It forms all or part of: allumette; elucidate; illumination; illustration; lea; leukemia; leuko-; light (n.) "brightness, radiant energy;" lightning; limn; link (n.2) "torch of pitch, tow, etc.;" lucent; lucid; Lucifer; luciferase; luciferous; lucifugous; lucubrate; lucubration; luculent; lumen; Luminal; luminary; luminate; luminescence; luminous; luna; lunacy; lunar; Lunarian; lunate; lunation; lunatic; lune; lunette; luni-; luster; lustrum; lux; pellucid; sublunary; translucent.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit rocate "shines;" Armenian lois "light," lusin "moon;" Greek leukos "bright, shining, white;" Latin lucere "to shine," lux "light," lucidus "clear;" Old Church Slavonic luci "light;" Lithuanian laukas "pale;" Welsh llug "gleam, glimmer;" Old Irish loche "lightning," luchair "brightness;" Hittite lukezi "is bright;" Old English leht, leoht "light, daylight; spiritual illumination," German Licht, Gothic liuhaþ "light."

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fruitcake (n.)
also fruit-cake, 1838 in the literal sense "a rich, sweet cake containing fruit," from fruit + cake (n.). Slang meaning "lunatic person" is first attested 1952.
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madman (n.)

"one who is insane, a lunatic," early 14c., mad man, from mad (adj.) + man (n.). One-word form attested from c. 1400, prevalent from 16c. Madwoman is by 1620s.

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madhouse (n.)

"lunatic asylum, house where insane persons are confined for cure or restraint," 1680s, from mad + house (n.). Figurative sense of "scene of uproar or confusion" is by 1919.

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bedlam (n.)

"scene of mad confusion," 1660s, from colloquial pronunciation of "Hospital of Saint Mary of Bethlehem" in London, founded 1247 as a priory, mentioned as a hospital 1330 and as a lunatic hospital 1402; it was converted to a civic lunatic asylum on dissolution of the monasteries in 1547. It was spelled Bedlem in a will from 1418, and Betleem is recorded as a spelling of Bethlehem in Judea from 971. The proper name might be caught in transition in the title of John Davies' 1617 publication of humorous poetry, "Wits bedlam —where is had, whipping-cheer, to cure the mad."

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