Etymology
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buy (n.)

"a purchase," especially a worthwhile one, 1875, American English, from buy (v.).

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buy (v.)

Middle English bien, from Old English bycgan (past tense bohte) "get by paying for, acquire the possession of in exchange for something of like value; redeem, ransom; procure; get done," from Proto-Germanic *bugjan (source also of Old Saxon buggjan, Old Norse byggja, Gothic bugjan), which is of unknown origin and not found outside Germanic.

The surviving spelling is southwest England dialect; the word was generally pronounced in Old English and Middle English with a -dg- sound as "budge," or "bidge." The meaning "believe, accept as true" is attested by 1926. Related: Bought; buying. To buy time "prevent further deterioration but make no improvement" is attested from 1946.

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up (n.)

"that which is up," 1530s, from up (adv.). Phrase on the up-(and-up) "honest, straightforward" first attested 1863, American English.

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buy-in (n.)

"act of obtaining an interest in," 1970, from verbal phrase buy in "to purchase a commission or stock" (1826), from buy (v.) + in (adv.). To buy into "obtain an interest in by purchase" (as of stock shares) is recorded from 1680s.

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up (adv.)

Old English up, uppe, from Proto-Germanic *upp- "up" (source also of Old Frisian, Old Saxon up "up, upward," Old Norse upp; Danish, Dutch op; Old High German uf, German auf "up"; Gothic iup "up, upward," uf "on, upon, under;" Old High German oba, German ob "over, above, on, upon"), from PIE root *upo "under," also "up from under," hence also "over."

As a preposition, "to a higher place" from c. 1500; also "along, through" (1510s), "toward" (1590s). Often used elliptically for go up, come up, rise up, etc. Up the river "in jail" first recorded 1891, originally in reference to Sing Sing, which is up the Hudson from New York City. To drive someone up the wall (1951) is from the notion of the behavior of lunatics or caged animals. Insulting retort up yours (scil. ass) is attested by late 19c.

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up- 

prefix with various senses, from Old English up (adv.), corresponding to similar prefixes in other Germanic languages.

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buy-out (n.)

also buy-out, "the purchasing of a controlling share in a company," 1961, from verbal phrase buy out "purchase (someone's) estate and turn him out of it," 1640s, from buy (v.) + out (adv.).

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up (v.)

1550s, "to drive and catch (swans)," from up (adv.). Intransitive meaning "get up, rise to one's feet" (as in up and leave) is recorded from 1640s. Sense of "to move upward" is recorded from 1737. Meaning "increase" (as in up the price of oil) is attested from 1915. Compare Old English verb uppian "to rise up, swell." Related: Upped; upping. Upping block, used for mounting or dismounting horses, carriages, etc., is attested from 1796 (earlier was horsing-block, 1660s).

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up (adj.)

c. 1300, "dwelling inland or upland," from up (adv.). Meaning "going up" is from 1784. From 1815 as "excited, exhilarated, happy," hence "enthusiastic, optimistic." Up-and-coming "promising" is from 1848. Musical up-tempo (adj.) is recorded from 1948.

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mark-up (n.)

also markup, "amount added by a retailer to cover overhead and provide profit," 1899, from the verbal phrase in this sense (by 1870); see mark (v.) + up (adv.).

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