Etymology
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light bulb (n.)
also lightbulb, 1884, from light (n.) + bulb (n.). Changing one as figurative of something easy to do is from 1920s; jokes about how many of a certain type it takes to change one date from 1971.
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sub-machine-gun (n.)

"light, portable machine gun," 1926, from sub- + machine-gun (n.).

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lucus a non lucendo 

a phrase that stands for "absurd etymology," or generally "anything illogical, outrageous hypothesis," 1711, from the Latin phrase, taken as the outstanding example of such an error.

"A grove (lucus) [is so called] from not (a non) being light" (lucendo, ablative of lucere "to shine;" see light (n.)). That is, it is called a grove because light doesn't get into it. This explanation is found in a commentary on Virgil (Aeneid 1.22) by Servius, a 4th century grammarian, among other places. Other ancient grammarians (notably Quintilian) found it paradoxical and absurd, based on nothing more than the similarity in sound between the two words.

Modern scholarship, however, concludes that lucus and lucere probably do come both from the same PIE root (*leuk-) meaning "light, bright." De Vaan writes: "Lucus 'sacred grove, wood,' from PIE *louk-o- 'light place,' with cognates in Sanskrit loka- 'free space, world,' Lithuanian laukas 'field, land,' Latvian lauks 'field, clearing in the woods,' Old High German loh 'clearing' and English lea 'open field, meadow, piece of untilled grassy ground.' " Apparently the primeval notion in *louk-o- was a lighter place in a thick forest. Migration, change of climate, or felling of the woods might have shifted the meaning.

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whole nine yards (n.)
by 1970, of unknown origin; perhaps arbitrary (see cloud nine). Among the guesses that have been made without real evidence: concrete mixer trucks were said to have dispensed in this amount. Or the yard might be the word used in the slang sense of "one hundred dollars." Several similar phrases meaning "everything" arose in the 1940s (whole ball of wax, which is likewise of obscure origin, whole schmear); older examples include whole hog (see hog (n.)) and whole shooting match (1896); whole shebang (1895).
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blind spot (n.)
1864, "spot within one's range of vision but where one cannot see," from blind (adj.) + spot (n.). Of the point on the retina insensitive to light (where the optic nerve enters the eye), from 1872. Figurative sense (of moral, intellectual, etc. sight) by 1907.
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glory hole (n.)
1825, "drawer or box where things are heaped together in a disorderly manner." The first element probably is a variant of Scottish glaur "to make muddy, dirty, defile" (Middle English glorien, mid-15c.), which is perhaps from Old Norse leir "mud." Hence, in nautical use, "a small room between decks," and, in mining, "large opening or pit." Meaning "opening through which the interior of a furnace may be seen and reached" (originally in glassblowing) is from 1849, probably from glory (n.), which had developed a sense of "circle or ring of light" by 1690s. Sexual (originally homosexual) sense from 1940s.
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silver lining (n.)

a "bright side" which proverbially accompanies even the darkest trouble; by 1843, apparently from oft-quoted lines from Milton's "Comus," where the silver lining is the light of the moon shining from behind the cloud.

Was I deceived? or did a sable cloud
Turn forth her silver lining on the night?
I did not err, there does a sable cloud,
Turn out her silver lining on the night
And casts a gleam over this tufted grove.

To which Thomas Warton added the commentary: "When all succour ſeems to be lost, Heaven unexpectedly presents the ſilver lining oſ a ſable cloud to the virtuous."

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fourth estate (n.)

"the press," by 1824, and especially from 1831, British English. For the other three, see estate. Earlier the term had been applied in various senses that did not stick, including "the mob" (1752), "the lawyers" (1825). The extension to the press is perhaps an outgrowth of the former.

Hence, through the light of letters and the liberty of the press, public opinion has risen to the rank of a fourth estate in our constitution; in times of quiet and order, silent and still, but in the collisions of the different branches of our government, deciding as an umpire with unbounded authority. ["Memoir of James Currie, M.D.," 1831]
[Newspapers] began to assume some degree of political importance, during the civil wars of the seventeenth century, in England; but it is not until within the last fifty years that they have become, — as they are now justly styled, — a Fourth Estate, exercising a more powerful influence on the public affairs of the countries in which they are permitted to circulate freely, than the other three put together. [Alexander H. Everett, "Address to the Phi Beta Kappa Society of Bowdoin College," 1834]
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