Etymology
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lightning (n.)
visible discharge of energy between cloud and cloud or cloud and ground, late Old English, "lightning, flash of lightning," verbal noun from lightnen "make bright," or else an extended form of Old English lihting, from leht (see light (n.)). The Old English word also meant "dawn, daybreak," and in Middle English "light of the sun, intense brightness, brilliance; the radiance of Christ." Another Middle English word for it was leven (mid-13c.), of uncertain origin, with no apparent source in Old English. (Old English had ligetung "lightning," from liget "lightning, flash of lightning." "Lightning" also was a specialized sense of lihting "lighting" and beorhtnes "brightness.")

Meaning "cheap, raw whiskey" is attested from 1781, also sometimes "gin." Lightning bug "firefly, phosphorescent beetle" is attested from 1778. Lightning rod from 1790.
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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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fulminate (v.)

early 15c., "publish a 'thundering' denunciation; hurl condemnation (at an offender)," a figurative use, from Latin fulminatus, past participle of fulminare "hurl lightning, lighten," figuratively "to thunder," from fulmen (genitive fulminis) "lightning flash," related to fulgor "lightning," fulgere "to shine, flash," from PIE *bhleg- "to shine, flash," from root *bhel- (1) "to shine, flash, burn." Metaphoric sense (the oldest in English) in reference to formal condemnation is from Medieval Latin fulminare, used of formal ecclesiastical censures. Related: Fulminated; fulminating.

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wildfire (n.)
late Old English wilde fyr "destructive fire" (perhaps caused by lightning); also "erysipelas, spreading skin disease;" see wild (adj.) + fire (n.). From c. 1300 as "Greek fire," also fire rained down from the sky as divine retribution. Figurative sense from late 14c. By 1795 as "sheet lightning."
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fulmination (n.)

c. 1500, "act of thundering forth denunciations," from French fulmination, from Latin fulminationem (nominative fulminatio) "a discharge of lightning," noun of action from past-participle stem of fulminare "to hurl lightning" (see fulminate). Literal sense "act of exploding or detonating" (1620s) is rare in English.

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lighting (n.)
"shining, illumination," Old English lihting "shining, illumination; dawn; lightning," from leoht (see light (n.)).
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jiffy (n.)
1785, "a moment, an instant, short space of time," colloquial, origin unknown; said to have been thieves' slang for "lightning."
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fulminant (adj.)
c. 1600, "fulminating, thundering," from French fulminant or directly from Latin fulminantem (nominative fulminans), present participle of fulminare "to hurl lightning" (see fulminate). As a noun from 1808.
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lighten (v.2)

"shed light upon, illuminate, make light or bright," early 14c., from light (n.) + -en (1). Intransitive meaning "to become brighter" is late 14c.; of faces, expressions, etc., from 1795. Meaning "to flash lightning" is from mid-15c. Related: Lightened; lightening.

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coup de foudre (n.)

"sudden, unforeseen occurrence," 1779, from French coup de foudre, literally "stroke of lightning," also "love at first sight" (see coup).

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