Etymology
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mustang (n.)

"small, half-wild horse of the American prairie and pampas," 1808, from Mexican Spanish mestengo "animal that strays" (16c.), from Spanish mestengo "wild, stray, ownerless," literally "belonging to the mesta," an association of cattle ranchers who divided stray or unclaimed animals that got "mixed" with the herds, from Latin mixta "mixed," fem. past participle of miscere "to mix" (from PIE root *meik- "to mix").

Said to be influenced by the Spanish word mostrenco "straying, wild," which is probably from mostrar, from Latin monstrare "to show." Though now feral, the animals are descended from tame horses brought to the Americas by the Spaniards. The brand of automobile was introduced by Ford in 1962.

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Main Street (n.)

"principal street of a (U.S.) town," 1810, from main (adj.) + street. Used allusively to indicate "mediocrity, small-town materialism" from late 19c., a sense reinforced by the publication of Sinclair Lewis's novel "Main Street" (1920).

But a village in a country which is taking pains to become altogether standardized and pure, which aspires to succeed Victorian England as the chief mediocrity of the world, is no longer merely provincial, no longer downy and restful in its leaf-shadowed ignorance. It is a force seeking to dominate the earth, to drain the hills and sea of color, to set Dante at boosting Gopher Prairie, and to dress the high gods in Klassy Kollege Klothes. Sure of itself, it bullies other civilizations, as a traveling salesman in a brown derby conquers the wisdom of China and tacks advertisements of cigarettes over arches for centuries dedicate to the sayings of Confucius. ["Main Street"]
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moor (n.)

"tract of open, untilled, more or less elevated ground, often overrun with heath," c. 1200, from Old English mor "morass, swamp," from Proto-Germanic *mora- (source also of Old Saxon, Middle Dutch, Dutch meer "swamp," Old High German muor "swamp," also "sea," German Moor "moor," Old Norse mörr "moorland," marr "sea"), perhaps related to mere (n.1), or from root *mer- "to die," hence "dead land."

The basic sense in place names is 'marsh', a kind of low-lying wetland possibly regarded as less fertile than mersc 'marsh.' The development of the senses 'dry heathland, barren upland' is not fully accounted for but may be due to the idea of infertility. [Cambridge Dictionary of English Place-Names]

Hence moor-fowl "grouse" (c. 1500); moor-hen (mid-14c.); moor-cock (c. 1200 as a surname).

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ambulance (n.)

1798, "mobile or field hospital," from French ambulance, formerly (hôpital) ambulant (17c.), literally "walking (hospital)," from Latin ambulantem (nominative ambulans), present participle of ambulare "to walk, go about" (see amble).

AMBULANCE, s. f. a moveable hospital. These were houses constructed in a manner so as to be taken to pieces, and carried from place to place, according to the movements of the army; and served as receptacles in which the sick and wounded men might be received and attended. ["Lexicographica-Neologica Gallica" (The Neological French Dictionary), William Dupré, London, 1801]

The word was not common in English until the meaning transferred from "field hospital" to "vehicle for conveying wounded from the field" (1854) during the Crimean War. It was extended early 20c. to vehicles to transport the sick or wounded in civilian life. In late 19c. U.S. the same word was used dialectally to mean "prairie wagon." Ambulance-chaser as a contemptuous term for a type of lawyer is by 1897.

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rolling 

14c. as a present-participle adjective from roll (v.), "that turns over and over, moving by means of rolling." The meaning "moving on wheels or as if on wheels" is by 1560s. Of thunder, etc., "making continuous noise," 1650s. The sense of "waving, undulating," of prairie land, etc., is from 1819. The meaning "staggered, rotating," of strikes, blackouts, etc., is by 1961.

From mid-15c. as a verbal noun. Rolling-pin "cylindrical piece of wood, etc., with a handle at each end, with which dough, etc. are reduced to proper thickness," is recorded from late 15c. Rolling-paper for cigarettes, etc., is by 1969. Rolling stock "wheeled vehicles on a railroad" (locomotives, carriages, etc.) is by 1853.

The rollyng stone neuer gatherth mosse. [John Heywood, "A dialogue conteinying the nomber in effect of all the proverbes in the Englishe tongue," 1546]

Hence figurative use of rolling stone, of persons, "a rambler, a wanderer" (1610s).

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timber (n.)

Old English timber "building, structure," in late Old English "building material, trees suitable for building," and "trees or woods in general," from Proto-Germanic *tem(b)ra- (source also of Old Saxon timbar "a building, room," Old Frisian timber "wood, building," Old High German zimbar "timber, wooden dwelling, room," Old Norse timbr "timber," German Zimmer "room"), from PIE *deme- "to build," possibly a form of the root *dem- meaning "house, household" (source of Greek domos, Latin domus).

The related Old English verb timbran, timbrian was the chief word for "to build" (compare Dutch timmeren, German zimmern). As a call of warning when a cut tree is about to fall, it is attested from 1912 in Canadian English. Timbers in the nautical slang sense (see shiver (v.2)) is from the specialized meaning "pieces of wood composing the frames of a ship's hull" (1748).

The timber-wolf (1846) of the U.S. West is the gray wolf, not confined to forests but so-called to distinguish it from the prairie-wolf (coyote).

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