Etymology
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dusk (n.)

"partial darkness, state between light and darkness, twilight," 1620s, from an earlier adjective dusk, from Middle English dosc (c. 1200) "obscure, not bright; tending to darkness, shadowy," having more to do with color than light, which is of uncertain origin, not found in Old English. Middle English also had it as a verb, dusken "to become dark." The Middle English noun was dusknesse "darkness" (late 14c.).

Perhaps it is from a Northumbrian variant of Old English dox "dark-haired, dark from the absence of light," with transposition of -k- and -s-, (compare colloquial ax for ask). But OED notes that "few of our words in -sk are of OE origin." Old English dox is from PIE *dus-ko- "dark-colored" (source also of Swedish duska "be misty," Latin fuscus "dark," Sanskrit dhusarah "dust-colored;" also compare Old English dosan "chestnut-brown," Old Saxon dosan, Old High German tusin "pale yellow").

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fuscous (adj.)
"dark-colored, of brown tinged with gray," 1660s, from Latin fuscus "dark, swarthy, dark-skinned" (see dusk). Earlier as fusc, fusk (1560s).
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dusky (adj.)

1550s, "somewhat dark, not luminous, dim;" see dusk + -y (2). "The normal source of an adj. in -y is a sb.; but the substantival use of dusk is not known so early as the appearance of dusky, so that the latter would appear to be one of the rare instances of a secondary adj. ..." [OED]. Meaning "rather black, dark-colored" is from 1570s. Related: Duskily; duskiness.

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obfuscate (v.)

"to darken, obscure, confuse, bewilder," 1530s, from Latin obfuscatus, past participle of obfuscare "to darken" (usually in a figurative sense), from ob "in front of, before" (see ob-) + fuscare "to make dark," from fuscus "dark" (see dusk). Related: Obfuscated; obfuscating.

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alpenglow (n.)
rose-colored light on high mountains before dawn or after dusk, 1871, translating German Alpenglühen; see Alp + glow (v.).
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fallow (adj.)

"pale yellow, brownish yellow," Old English fealu "reddish yellow, yellowish-brown, tawny, dusk-colored" (of flame, birds' feet, a horse, withered grass or leaves, waters, roads), from Proto-Germanic *falwa- (source also of Old Saxon falu, Old Norse fölr, Middle Dutch valu, Dutch vaal, Old High German falo, German falb), from PIE root *pel- (1) "pale." Related: Fallow-deer.

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lit (n.1)
"color, hue, dye," early 12c., from Old Norse litr "color, hue; the color of the sky at dawn or dusk," from Proto-Germanic *wlitiz (source also of Old Frisian wlite "exterior, form," Gothic *wlits "face, form"). The cognate Old English word was wlite "brightness; appearance, form, aspect; look, countenance; beauty, splendor," which seems to have been rare after c. 1400. Compare litmus.
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Gotterdammerung (n.)

1909 in the figurative sense of "complete overthrow" of something; from German Götterdämmerung (18c.), literally "twilight of the gods," from genitive plural of Gott "god" (see god) + Dämmerung "dusk, twilight," from PIE root *teme- "dark" (see temerity). Used by Wagner as the title of the last opera in the Ring cycle. It translates Old Norse ragna rok "the doom or destruction of the gods, the last day, world's end." A better transliteration is Goetterdaemmerung.

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Samarra 

city in north-central Iraq; phrase an appointment in Samarra indicating the inevitability of death is from an old Arabic tale (first told in English apparently in W. Somerset Maugham's play "Sheppey," 1933): One day in the marketplace in Baghdad a man encounters Death (with a surprised look on his bony face); the man flees in terror and by dusk has reached Samarra. Death casually takes him there, and, when questioned, says, "I was astonished to see him in Baghdad, for I had an appointment with him tonight in Samarra."

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crepuscular (adj.)

figurative use, "dim, indistinct," is attested from 1660s; literal use, "pertaining to or resembling twilight," from 1755, from Latin crepusculum "twilight, dusk," related to creper "obscure, uncertain," from Proto-Italic *krepos "twilight," which is of uncertain origin. It is not certain whether "twilight" or "obscure" was the original sense; de Vaan writes, "there is no known root of the form *krep- from which the extant meanings can be derived."

Especially of evening twilight, but 17c.-18c. also "like morning twilight" as symbolic of imperfect enlightenment. In zoology, "flying or appearing at sunset," from 1826. An older (and lovelier-sounding) adjective form was crepusculine (1540s).

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