Etymology
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fleeting (adj.)
early 13c., "fickle, shifting, unstable," from Old English fleotende "floating, drifting," later "flying, moving swiftly," from present participle of fleotan "to float, drift, flow" (see fleet (v.)). Meaning "existing only briefly" is from 1560s. Related: Fleetingly.
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fleet (v.)
Old English fleotan "to float; drift; flow, run (as water); swim; sail (of a ship)," from Proto-Germanic *fleutan (source also of Old Frisian fliata, Old Saxon fliotan "to flow," Old High German fliozzan "to float, flow," German fliessen "to flow, run, trickle" (as water), Old Norse fliota "to float, flow"), from PIE root *pleu- "to flow."

Meaning "to glide away like a stream, vanish imperceptibly" is from c. 1200; hence "to fade, to vanish" (1570s). Related: Fleeted; fleeting.
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*pleu- 
Proto-Indo-European root meaning "to flow."

It forms all or part of: fletcher; fledge; flee; fleet (adj.) "swift;" fleet (n.2) "group of ships under one command;" fleet (v.) "to float, drift; flow, run;" fleeting; flight (n.1) "act of flying;" flight (n.2) "act of fleeing;" flit; float; flood; flotsam; flotilla; flow; flue; flugelhorn; fluster; flutter; fly (v.1) "move through the air with wings;" fly (n.) "winged insect;" fowl; plover; Pluto; plutocracy; pluvial; pneumo-; pneumonia; pneumonic; pulmonary.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit plavate "navigates, swims;" Greek plynein "to wash," plein "to navigate," ploein "to float, swim," plotos "floating, navigable," pyelos "trough, basin;" Latin plovere "to rain," pluvius "rainy;" Armenian luanam "I wash;" Old English flowan "to flow;" Old Church Slavonic plovo "to flow, navigate;" Lithuanian pilu, pilti "to pour out," plauju, plauti "to swim, rinse."
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caducous (adj.)
"having a tendency to fall or decay," 1797, in botany, from Latin caducus "falling, fallen, fleeting," from cadere "to fall, decline, perish," from PIE root *kad- "to fall." Related: Caducity.
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fugacious (adj.)
"fleeing, likely to flee," 1630s, with -ous + Latin fugaci-, stem of fugax "apt to flee, timid, shy," figuratively "transitory, fleeting," from fugere "to flee" (see fugitive). Related: Fugaciously; fugaciousness; fugacity.
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shadowy (adj.)

late 14c., shadwi, shadewy, "full of shadows, shaded," also "transitory, fleeting, unreal" ("resembling a shadow); see shadow (n.) + -y (2). From 1797 as "faintly perceptible." Related: Shadowiness. Old English had sceadwig "shady." Caxton offers shadowous.

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lucifugous (adj.)
"shunning light" (in reference to bats, cockroaches, etc.), 1650s, from Latin lucifugus "light-shunning," from stem of lucere "to shine" (from suffixed (iterative) form of PIE root *leuk- "light, brightness") + stem of fugax "apt to flee, timid," figuratively "transitory, fleeting," from fugere "to flee" (see fugitive (adj.)).
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volatile (adj.)

1590s "fine or light," also "evaporating rapidly" (c. 1600), from French volatile, from Latin volatilis "fleeting, transitory; swift, rapid; flying, winged," from past participle stem of volare "to fly" (see volant). Sense of "readily changing, flighty, fickle" is first recorded 1640s. Volatiles in Middle English meant "birds, butterflies, and other winged creatures" (c. 1300).

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floricide (n.)

"one who destroys flowers," 1841, from Latin floris, genitive of flos "flower" (see flora) + -cide "killer."

[S]urely there is cruelty and gross selfishness in cutting down for our own fleeting gratification that which would have ministered to the enjoyments of all for weeks or months. Frankly do I confess that I dislike a wanton floricide. He has robbed the world of a pleasure; he has blotted out a word from God's earth-written poetry. [New Monthly Magazine, April 1847]
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may (v.2)

"to celebrate May Day, to take part in May Day festivities," late 15c., from May. Maying as "the observance of May Day with all its sports and games" is attested from late 14c. (maiing).

And as a vapour, or a drop of raine
Once lost, can ne'r be found againe:
                     So when or you or I are made
                     A fable, song, or fleeting shade;
                     All love, all liking, all delight
                     Lies drown'd with us in endlesse night.
Then while time serves, and we are but decaying;
Come, my Corinna, come, let's goe a Maying.
[Robert Herrick, "Corinna's Going a-Maying," 1648] 
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