Etymology
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melt (n.)

1854, "molten metal, a substance in a melted condition," from melt (v.). In reference to a type of sandwich (typically tuna) topped by melted cheese, by 1956, American English.

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melt (v.)

Middle English melten, from Old English meltan (intransitive) "become liquid through heat" (class III strong verb; past tense mealt, past participle molten), from Proto-Germanic *meltanan; fused with Old English gemæltan (Anglian), gemyltan (West Saxon) "make liquid, reduce from a solid to a fluid state by means of heat" (transitive), from Proto-Germanic *gamaltijan (source also of Old Norse melta "to digest").

Both Germanic words are from PIE *meldh- (source also of Sanskrit mrduh "soft, mild," Greek meldein "to melt, make liquid," Latin mollis "soft, mild"), from root *mel- (1) "soft." Also in Middle English "dissolve" (of salt, sugar, etc.), "corrode" (of iron), "putrefy" (of flesh). Meaning "pass imperceptibly from one thing into another" is by 1781. Related: Melted; melting.

Figurative use "to diminish, wane; be touched, grow tender" is by c. 1200; transitive sense of "soften" (to love, pity, tenderness) is by early 14c. Of food, to melt in (one's) mouth is from 1690s. Melting point "degree of temperature at which a solid body melts" is by 1807. Melting pot is from early 15c.; figurative use from 1855; popularized with reference to immigrant assimilation in the United States by the play "The Melting Pot" by Israel Zangwill (1908):

DAVID Yes, East and West, and North and South, the palm and the pine, the pole and the equator, the crescent and the cross—how the great Alchemist melts and fuses them with his purging flame! Here shall they all unite to build the Republic of Man and the Kingdom of God. Ah, Vera, What is the glory of Rome and Jerusalem where all nations and races come to worship and look back, compared with the glory of America where all races and nations come to labour and look forward!
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down (adj.)

1560s, "directed downward," from down (adv.). Sense of "depressed mentally" is attested from c. 1600. Slang sense of "aware, wide awake" is attested from 1812. Computer crash sense is from 1965. Down-and-out "completely without resources" is from 1889, American English, from situation of a beaten prizefighter.

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down (n.3)

1710, "a downward movement," from down (adv.). Football sense of "an attempt to advance the ball" is by 1882.

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down (adv.)

"in a descending direction, from a higher to a lower place, degree, or condition," late Old English shortened form of Old English ofdune "downwards," originally of dune "off from (the) hill," from dune "from the hill," dative of dun "hill" (see down (n.2)). The "hill" word is general in Germanic, but this sense development is peculiar to English. As a preposition, "in a descending direction upon or along,"  from late 14c.

To be down on "express disapproval of" is by 1851. Down home is from 1828 as "in one's home region," as an adjective phrase meaning "unpretentious" by 1931, American English. Down the hatch as a toast is from 1931. Down to the wire is 1901, from horse-racing.

Down Under "Australia and New Zealand" attested from 1886; Down East "Maine" is from 1825; Down South "in the Southern states of the U.S." is attested by 1834. Down the road "in the future" is by 1964, U.S. colloquial. Down-to-earth "everyday, ordinary, realistic" is by 1932.

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down (v.)

1560s, "cause to go down," from down (adv.). Meaning "swallow hastily" is by 1860; football sense of "bring down (an opposing player) by tackling" is attested by 1887. Figurative sense of "defeat, get the better of" is by 1898. Related: Downed; downing.

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down (n.1)

"first feathers of a baby bird; soft covering of fowls under the feathers, the under-plumage of birds," used for stuffing pillows and feather-beds, mid-14c., from Old Norse dunn, which is of uncertain origin. Extended in Modern English to the soft hair of the human face and fine soft pubescence upon plants and some fruit.

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down (n.2)

"a hill of moderate elevation and more or less rounded outline," Old English dun "height, hill, moor," from Proto-Germanic *dunaz- (source also of Middle Dutch dunen "sandy hill," Dutch duin), "probably a pre-insular loan-word from Celtic" [Cambridge Dictionary of English Place-Names], in other words, borrowed at a very early period, before the Anglo-Saxon migration, perhaps from PIE root *dheue- "to close, finish, come full circle."

The more general meaning "elevated rolling grassland; high, rolling region not covered by forest" is from c. 1400. Specifically of certain natural pastureland districts of south and southeast England (the Downs) by mid-15c.

The non-English Germanic words tend to mean "dune, sand bank" (see dune), while the Celtic cognates tend to mean "hill, citadel" (compare Old Irish dun "hill, hill fort;" Welsh din "fortress, hill fort;" and second element in place names London, Verdun, etc.). German Düne, French dune, Italian, Spanish duna are said to be loan-words from Dutch.

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down-hearted (adj.)

also downhearted, "dejected, depressed, discouraged," 1774 (downheartedly is attested from 1650s), a figurative image from down (adv.) + -hearted.

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down-market (adj.)

"on the cheaper end of what is available," 1970, from down (adj.) + market (n.).

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