Online Etymology Dictionary

lumbaginous (adj.): 1620s, from Latin lumbagin-, from lumbago (see lumbago) + -ous.
lumbago (n.): 1690s, from Late Latin lumbago "weakness of loins and lower back," from Latin lumbus "loin" (usually plural), from PIE *lendh- "loin" (cf. Old Church Slavonic ledvije (plural) "loins; soul," Russian ljadveja "loin;" Old English lendenu "loins," German Lende "loin," Lenden "loins").
lumbar (adj.): "pertaining to or situated near the loins," 1650s, from Modern Latin lumbaris, from Latin lumbus "loin" (see lumbago).
lumber (n.): "timber sawn into rough planks," 1660s, American English (Massachusetts), earlier "disused bit of furniture; heavy, useless objects" (1550s), probably from lumber (v.), perhaps influenced by Lombard, from the Italian immigrants famous as pawnbrokers and money-lenders in England (see Lombard). Lumbar, Lumbard were old alternative forms of Lombard in English. The evolution of sense then would be because a lumber-house ("pawn shop") naturally accumulates odds and ends of furniture.
Live Lumber; soldiers or passengers on board a ship are so called by the sailors.

LUMBER HOUSE. A house appropriated by thieves for the reception of their stolen property. ["Dictionary of Buckish Slang, University Wit, and Pickpocket Eloquence," London, 1811]
lumber (v.): "to move clumsily," c.1300, lomere, probably from a Scandinavian source (cf. dialectal Swedish loma "move slowly, walk heavily," Old Norse lami "lame"), ultimately cognate with lame (adj.). Related: Lumbered; lumbering.
lumberjack (n.): 1831, Canadian English, from lumber (n.) + Jack.
lumbo-: word-forming element meaning "loin, loins," from comb. form of Latin lumbus (see lumbago).
lumen (n.): unit of luminosity, 1897, coined 1894 by French physicist André-Eugène Blondel (1863-1938) from Latin lumen "light," related to lucere "to shine" (see light (n.)).
Lumiere: in reference to the early color photography process, from the names of French brothers Auguste (1862-1954) and Louis (1864-1948) Lumière, photographers. The name is literally "light, lamp."
luminaire (n.): electric light, 1921, from French luminaire, from Old French luminarie (see luminary).
Luminal (n.): trade name of phenobarbitone, used as a sedative and hypnotic, coined 1912 in German from Latin lumen "light," related to lucere "to shine" (see light (n.)), + -al (3), "the root here being used, very irregularly, as an equivalent of pheno- [Flood].
luminal (adj.): "of or pertaining to a lumen," 1897, with -al (1).
luminance (n.): "luminousness," 1862, from Latin luminantem (nominative luminans), present participle of luminare (see luminary).
luminary (n.): mid-15c., "lamp, source of (artificial) light," from Old French luminarie (12c.), "lamp, lights, lighting; candles; brightness, illumination," from Late Latin luminare "light, torch, lamp, heavenly body," literally "that which gives light," from Latin lumen (genitive luminis) "light," related to lucere "to shine" (see light (n.)). Sense of "notable person" is first recorded 1690s, though the Middle English word also had a figurative sense of "source of spiritual light, example of holiness."
luminate (v.): 1620s (now obsolete), from Latin luminatus, past participle of luminare (see luminary). Related: Luminated; luminating.
luminescence (n.): 1884, from Latin lumen (genitive luminis) "light" (see luminous) + -escence.
Fluorescence and Phosphorescence -- Prof. E. Wiedmann has made a new study of these phenomena. He proposes the general name luminescence for evolutions of light which do not depend on the temperature of the substance concerned. ["Photographic News," April 20, 1888]
luminescent: 1889, from luminescence + -ent.
luminosity (n.): 1630s, "quality of being luminous," from French luminosité or else a native formation from luminous + -ity. In astronomy, "intrinsic brightness of a heavenly body" (as distinguished from apparent magnitude, which diminishes with distance), attested from 1906.
luminous (adj.): early 15c., "full of light," from Latin luminosus "shining, full of light," from lumen (genitive luminis) "light," related to lucere "to shine" (see light (n.)). Related: Luminously.
lummox (n.): 1825, East Anglian slang, perhaps from dumb ox, influenced by lumbering; or from E. Anglian dialectal form of verb lummock "move heavily or clumsily," of uncertain origin.
lump (n.): early 14c., lumpe (1224 as surname), probably in Old English, perhaps from a Scandinavian source (cf. cognate Danish lumpe, 16c.), of unknown origin. Cf. also Middle High German lumpe, early modern Dutch lompe. Phrase lump in (one's) throat "feeling of tightness brought on by emotion" is from 1803. Lumps "hard knocks, a beating" is colloquial, from 1934. Lump sum, one covering a number of items, is from 1867.
lump (v.2): "endure" (now usually in contrast to like), 1791, apparently an extended sense from an older meaning "to look sulky, dislike" (1570s), of unknown origin, perhaps a symbolic sound (cf. grump, harumph, etc.). Related: Lumped; lumping.
LUMPING. Great. A lumping pennyworth; a great qualtity for the money, a bargain. He has got a lumping pennyworth; frequently said of a man who marries a fat woman. ["Dictionary of Buckish Slang, University Wit, and Pickpocket Eloquence," London, 1811]
lump (v.1): early 15c., "to curl up in a ball, to gather into a lump" (implied in lumped), from lump (n.). Meaning "to put together in one mass or group" is from 1620s. Related: Lumped; lumping.
lumpectomy (n.): 1971, from lump (n.), coined on model of mastectomy.
lumpenproletariat (n.): 1897, from German Lumpenproletariat (1850), from Karl Marx, who coined it to mean "rabble, poorest of the working class," from German lump "ragamuffin" + proletariat. Secondary sense of "boorish, stupid people" led to lumpen- being taken as a suffix meaning "unenlightened."
lumpy (adj.): 1707, from lump (n.) + -y (2). Related: Lumpiness.
Luna (n.): late 14c. "moon," also an alchemical name for "silver;" from Latin luna "moon, goddess of the moon," from *leuksna- (cf. Old Church Slavonic luna "moon," Old Prussian lauxnos "stars," Middle Irish luan "light, moon"), from the same source as lux, lumen "light," lucere "to shine" (see light (n.)). The luna moth (1841, American English) so called for the crescent-shaped markings on its wings. Lunarian (1708) was an early word for "inhabitant of the moon."
lunacy (n.): 1540s, "condition of being a lunatic," formed in English from lunatic + -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."
lunar (adj.): "crescent-shaped," early 15c.; "pertaining to the moon," 1620s, from Old French lunaire (15c.), from Latin lunaris "of the moon," from luna "moon," (with capital L- "moon goddess"); see Luna.
lunate (adj.): "crescent-shaped," 1777, from Latin lunatus, from luna (see Luna).
lunatic (adj.): late 13c., "affected with periodic insanity, dependent on the changes of the moon," from Old French lunatique, lunage "insane," or directly from Late Latin lunaticus "moon-struck," from Latin luna "moon" (see Luna). Cf. Old English monseoc "lunatic," literally "moon-sick;" Middle High German lune "humor, temper, mood, whim, fancy" (German Laune), from Latin luna. Cf. 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."
lunatic (n.): "lunatic person," late 14c., from lunatic (adj.).
lunation (n.): "time from one new moon to another," late 14c., from Medieval Latin lunationem, from luna "moon" (see Luna).
lunch (n.): "mid-day repast," 1786, shortened form of luncheon (q.v.). The verb meaning "to take to lunch" (said to be from the noun) also is attested from 1786:
PRATTLE. I always to be ſure, makes a point to keep up the dignity of the family I lives in. Wou'd you take a more ſolid refreſhment?--Have you lunch'd, Mr. Bribe?

BRIBE. Lunch'd O dear! Permit me, my dear Mrs. Prattle, to refreſh my sponge, upon the honey dew that clings to your raviſhing pouters. O! Mrs. Prattle, this ſhall be my lunch. (kiſſes)

["The Mode," in William Davies' "Plays Written for a Private Theatre," London, 1786]
But as late as 1817 the only definition of lunch in Webster's is "a large piece of food." OED says in 1820s the word "was regarded either as a vulgarism, or as a fashionable affectation." Related: Lunched; lunching. Lunch money is attested from 1868; lunch-time (n.) is from 1821; lunch hour is from 1840. Slang phrase out to lunch "insane, stupid, clueless" first recorded 1955, on notion of being "not there." Old English had nonmete "afternoon meal," literally "noon-meat."
luncheon (n.): "light repast between mealtimes," 1650s (lunching; spelling luncheon by 1706); earlier "thick piece, hunk," 1570s (luncheon), of uncertain origin. Perhaps northern English dialectal lunch "hunk of bread or cheese" (1580s; probably from Spanish lonja "a slice," literally "loin"), blended with or influenced by nuncheon (Middle English nonechenche, mid-14c.) "light mid-day meal," from none "noon" (see noon) + schench "drink," from Old English scenc, from scencan "pour out."

Despite the form lunching in the 1650s source OED discounts that it possibly could be from lunch (v.), which is much later. It suggests perhaps an analogy with truncheon, etc. Especially in reference to an early afternoon meal eaten by those who have a noontime dinner.
luncheonette (n.): type of restaurant, 1906, American English, from luncheon + diminutive ending -ette.
lune (n.): figure formed by two arcs of circles, 1704, from Latin luna "moon; crescent-shaped badge" (see luna).
lunette (n.): 1570s, "semi-circular horseshoe," from Middle French lunette (13c.), literally "little moon," diminutive of lune "moon," from Latin luna (see luna). Later applied to a wide range of objects and ornamentations resembling a crescent moon.
lung (n.): "human respiratory organ," c.1300, from Old English lungen (plural), from Proto-Germanic *lungw- (cf. Old Norse lunge, Old Frisian lungen, Middle Dutch longhe, Dutch long, Old High German lungun, German lunge "lung"), literally "the light organ," from PIE *legwh- "not heavy, having little weight; easy, agile, nimble" (cf. Russian lëgkij, Polish lekki "light;" Russian lëgkoje "lung," Greek elaphros "light" in weight; see also lever).

The notion probably is from the fact that, when thrown into a pot of water, lungs of a slaughtered animal float, while the heart, liver, etc., do not. Cf. also Portuguese leve "lung," from Latin levis "light;" Irish scaman "lungs," from scaman "light;" Welsh ysgyfaint "lungs," from ysgafn "light." See also lights, pulmonary. Lung cancer attested from 1882.
lunge (n.): 1735, "a thrust with a sword," originally a fencing term, shortened from allonge, from French allonger "to extend, thrust," from Old French alongier "to lengthen, make long," from à "to" + Old French long, from Latin longus "long" (see long (adj.)).
lunge (v.): 1735 (implied in lunged), from lunge (n.). Sense of "to make a sudden forward rush" is from 1821. Related: Lunged; lunging.
lungfish (n.): 1883, from lung + fish (n.).
lunk (n.): "slow-witted person," 1867, American English colloquial, shortened from lunkhead (1852), possibly an altered form of lump (n.) + head (n.)
Lupercalia (n.): Roman festival held Feb. 15, in honor of Lupercus, god (identified with Lycean Pan) who had a grotto at the foot of the Palatine Hill, from Latin Lupercalia (plural), from Lupercalis "pertaining to Lupercus," whose name derives from lupus "wolf" (see wolf (n.)).
lupin (n.): plant of the genus lupinus, late 14c., from Latin lupinus, name of the plant, noun use of an adjective meaning "of a wolf" (see lupine). The reason for association with the animal is unclear; perhaps it was so called because of a belief that the plants were harmful to soil (cf. lupus).
lupine (adj.): "wolf-like," 1650s, from French lupine "wolf-like," from Latin lupinus "of the wolf," from lupus "wolf" (see wolf (n.)).
lupus (n.): late 14c., used of several diseases that cause ulcerations of the skin, from Medieval Latin lupus, from Latin lupus "wolf" (see wolf (n.)), apparently because it "devours" the affected part.
lurch (n.1): "sudden pitch to one side," 1784, from earlier lee-larches (1765), a nautical term for "the sudden roll which a ship makes to lee-ward in a high sea, when a large wave strikes her, and bears her weather-side violently up, which depresses the other in proportion" ["Complete Dictionary of Arts and Sciences," London 1765]; perhaps from French lacher "to let go," from Latin laxus (see lax).
When a Ship is brought by the Lee, it is commonly occaſsioned by a large Sea, and by the Neglect of the Helm's-man. When the Wind is two or three Points on the Quarter, the Ship taking a Lurch, brings the Wind on the other Side, and lays the Sails all dead to the Maſt; as the Yards are braced up, ſhe then having no Way, and the Helm being of no Service, I would therefore brace about the Head ſails ſharp the other Way .... [John Hamilton Moore, Practical Navigator, 8th ed., 1784]
lurch (n.2): "predicament," 1580s, from Middle English lurch (v.) "to beat in a game of skill (often by a great many points)," mid-14c., probably literally "to make a complete victory in lorche," a game akin to backgammon, from Old French lourche. The game name is perhaps related to Middle English lurken, lorken "to lie hidden, lie in ambush," or it may be adopted into French from Middle High German lurz "left," also "wrong."
lurch (v.): 1821, from lurch (n.1). Related: Lurched; lurching.