Etymology
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bustle (v.)
"be active in a noisy and agitated way," 1570s (bustling "noisy or excited activity" is from early 15c.), of uncertain origin, perhaps a frequentative of Middle English bresten "to rush, break," from Old English bersten (see burst (v.)), influenced by Old Norse buask "to make oneself ready" (see busk (v.)). Or it might be from busk (v.) via a 16c. frequentative form buskle. Related: Bustled; bustling; bustler.
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bustle (n.2)

"padding in the upper back part of a skirt," 1788, of uncertain origin, perhaps from German Buschel "bunch, pad," or it might be a special use of bustle (n.1) with reference to "rustling motion."

BUSTLE. A pad stuffed with cotton, feathers, bran, &c., worn by ladies for the double purpose of giving a greater rotundity or prominence to the hips, and setting off the smallness of the waist. [Bartlett, "Dictionary of Americanisms," 1848]

Century Dictionary (1895) notes that, in addition to "improving the figure" it causes the folds of the skirt to hang gracefully and prevents the skirt from interfering with the feet in walking.

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bustle (n.1)
"activity, stir, fuss, commotion," 1630s (Milton), from bustle (v.).
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bustling (adj.)
of a place, "noisily active," 1819, present-participle adjective from bustle (v.).
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rustle (v.)

"to emit soft, rapid sounds when in motion," late 14c. (implied in rustling "moving about noisily"), a word of uncertain origin, perhaps imitative (compare Middle Low German ruschen, Middle Dutch ruusscen, German rauschen "to rustle"). Related: Rustled; rustling.

The meaning "steal" (especially cattle) is attested by 1882, and is probably from earlier Western U.S. slang rustle "make, do, secure, etc. in a vigorous way" (1844), which is perhaps a separate word, compounded from rush and hustle. Compare sense developments in bustle (v.), hustle (v.), and compare rustler. To rustle up (transitive) in the general sense of "gather up, round up" is by 1896. 

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*bheue- 

*bheuə-, also *bheu-, Proto-Indo-European root meaning "to be, exist, grow."

It forms all or part of: Bauhaus; be; beam; Boer; bondage; boodle; boom (n.1) "long pole;" boor; booth; bound (adj.2) "ready to go;" bower; bowery; build; bumpkin; busk; bustle (v.) "be active;" byre; bylaw; Eisteddfod; Euphues; fiat; forebear; future; husband; imp; Monophysite; neighbor; neophyte; phyletic; phylo-; phylum; phylogeny; physic; physico-; physics; physio-; physique; -phyte; phyto-; symphysis.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit bhavah "becoming," bhavati "becomes, happens," bhumih "earth, world;" Greek phyein "to bring forth, make grow," phytos, phyton "a plant," physis "growth, nature," phylon "tribe, class, race," phyle "tribe, clan;" Old English beon "be, exist, come to be, become, happen;" Old Church Slavonic byti "be," Greek phu- "become," Old Irish bi'u "I am," Lithuanian būti "to be," Russian byt' "to be."

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fluster (v.)
early 15c. (implied in flostrynge), "bluster, agitate," probably from a Scandinavian source (compare Icelandic flaustr "bustle," flaustra "to bustle"), from Proto-Germanic *flaustra-, probably from PIE *pleud-, extended form of root *pleu- "to flow." Originally "to excite," especially with drink; sense of "to flurry, confuse" is from 1724. Related: Flustered; flustering; flustery. As a noun, 1710, from the verb.
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tumultuous (adj.)

1540s, from French tumultuous (Modern French tumultueux), from Latin tumultuosus "full of bustle or confusion, disorderly, turbulent," from tumultus (see tumult). Related: Tumultuously; tumultuousness.

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tumult (n.)
late 14c., from Old French tumult (12c.), from Latin tumultus "commotion, bustle, uproar, disorder, disturbance," related to tumere "to be excited, swell" (from PIE root *teue- "to swell").
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field-day (n.)
1747, originally a day of military exercise and review (see field (v.)); figurative sense "any day of unusual bustle, exertion, or display" [Century Dictionary] is from 1827.
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