Etymology
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upside down (adv.)
late 15c., earlier upsadoun (late 14c.), up so down (c. 1300); the so perhaps meaning "as if." As an adjective from 1866.
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downscale (v.)

"reduce in size or scale," 1945, American English, from down (adv.) + scale (v.). In business, especially, "to reduce the size of an operation." Related: Downscaled; downscaling. From 1966 as an adjective.

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downstream (adv.)

"with or in the direction of the current of a stream," 1706, from the prepositional phrase; see down (adv.) + stream (n.). As an adjective by 1842. Middle English had the prepositional phrase down the water (c. 1400).

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downplay (v.)

"de-emphasize, minimize," 1968, from verbal phrase play (something) down, which is perhaps from music or theater;  down (adv.) + play (v.). Related: Downplayed; downplaying.

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downfall (n.)

early 14c., "ruin, fall from high condition, complete overthrow," from down (adv.) + fall (v.). From c. 1500 as "a falling downward." Verbal phrase fall down in the sense of "go to ruin" is attested from late 12c.

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downhill (adv.)

"in a descending direction," late 14c., from down (adv.) + hill (n.).  From 1590s as a noun, "downward slope of a hill;" meaning "downhill skiing race" is from 1960. As an adjective, "sloping downward, descending," from 1727.

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sundown (n.)
also sun-down, 1610s, from sun (n.) + down (adv.). OED suggests perhaps a shortening of sun-go-down (1590s). Compare sunset.
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lockdown (n.)
also lock-down, from late 19c. in various mechanical senses, from the verbal phrase; see lock (v.) + down (adv.). Prison sense is by 1975, American English.
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adown (adv.)
"to a lower place," Old English adune (adv.), originally a prepositional phrase, of dune "down, downward;" see a- (1) + down (adv.).
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downtown 

1835 as an adverb, "into the town," from the prepositional phrase; see down (adv.) + town. The notion is originally literal, of suburbs built on heights around a city. From 1836 as an adjective; by 1851 as a noun.

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