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."
1630s, "authoritative sanction," from Latin fiat "let it be done" (used in the opening of Medieval Latin proclamations and commands), third person singular present subjunctive of fieri "be done, become, come into existence" (from PIE root *bheue- "to be, exist, grow"), used as passive of facere "to make, do." Meaning "a decree, command, order" is from 1750. In English the word also sometimes is a reference to fiat lux "let there be light" in Genesis i.3.
Dixitque Deus: Fiat lux. Et facta est lux. [Vulgate]
early 15c., "luminous, bright;" 1590s, "evident, lucid," from Latin luculentus "full of light, bright, splendid," from the stem of lux "light" (from PIE root *leuk- "light, brightness").
Sense of "easy to understand, free from obscurity of meaning, marked by intellectual clarity" first recorded 1786. Lucid interval "period of calm or temporary sanity" (1580s) is from Medieval Latin lucida intervalla (plural), common in medieval legal documents (non est compos mentis, sed gaudet lucidis intervallis, etc.). The notion probably is of a period of calm and clear during a storm. Related: Lucidly; lucidness (1640s).
In 1922, Lever Brothers began to advertise Lux in this country to "rid your hands of that dishpan look." Without any break since then the company has alluded to "dishpan hands" which come from using soap that is too strong in alkaline content. [Printers' Ink, vol. 173, 1935]
c. 1300, "sexual intercourse;" mid-14c., "lasciviousness, sinful self-indulgence;" late 14c., "sensual pleasure," from Old French luxurie "debauchery, dissoluteness, lust" (12c., Modern French luxure), from Latin luxuria "excess, extravagant living, profusion; delicacy" (source also of Spanish lujuria, Italian lussuria), from luxus "excess, extravagance; magnificence," probably a figurative use of luxus (adj.) "dislocated," which is related to luctari "wrestle, strain" (see reluctance).
The English word lost its pejorative taint 17c. Meaning "habit of indulgence in what is choice or costly" is from 1630s; that of "sumptuous surroundings" is from 1704; that of "something choice or comfortable beyond life's necessities" is from 1780. Used as an adjective from 1916.
In Lat. and in the Rom. langs. the word connotes vicious indulgence, the neutral sense of the Eng. 'luxury' being expressed by L. luxus, F. luxe, Sp. lujo, It. lusso. [OED]