Etymology
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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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buy (n.)

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

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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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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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bought 

past tense and past participle of buy (v.).

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buyer (n.)

c. 1200, biggere "one who purchases," agent noun from buy (v.). The meaning "one whose job is to buy goods for a store" is from 1884. Buyer's market attested from 1886.

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boughten (adj.)

irregular past participle of buy, attested as an adjective from 1793, especially in colloquial U.S. usage, in reference to clothing and other items, and opposed to made.

BOUGHTEN. Which is bought. This is a common word in the interior of New England and New York. It is applied to articles purchased from the shops, to distinguish them from similar articles of home manufacture. [Bartlett, "Dictionary of Americanisms," 1848]
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sumptuary (adj.)

"pertaining to expense," c. 1600, from Latin sumptuarius "relating to expenses," from sumptus "expense, cost," from sumere "to borrow, buy, spend, eat, drink, consume, employ, take, take up," contraction of *sub-emere, from sub "under" (see sub-) + emere "to take, buy" (from PIE root *em- "to take, distribute").

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*em- 

Proto-Indo-European root meaning "to take, distribute." 

It forms all or part of: assume; consume; emption; example; exemplar; exemplary; exemplify; exempt; exemption; impromptu; peremptory; pre-emption; premium; presume; presumption; prompt; pronto; ransom; redeem; redemption; resume; sample; sejm; subsume; sumptuary; sumptuous; vintage.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit yamati "holds, subdues;" Latin emere "buy," originally "take," sumere "to take, obtain, buy;" Old Church Slavonic imo "to take;" Lithuanian imu, imti "to take."

For the sense shift from "take" to "buy" in the Latin verbs, compare Old English sellan "to give," source of Modern English sell "to give in exchange for money;" Hebrew laqah "he bought," originally "he took;" and colloquial English I'll take it for "I'll buy it." 

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sumptuous (adj.)

late 15c., from Old French sumptueux or directly from Latin sumptuosus "costly, very expensive; lavish, wasteful," from sumptus, past participle of sumere "to borrow, buy, spend, eat, drink, consume, employ, take, take up," contraction of *sub-emere, from sub "under" (see sub-) + emere "to take, buy" (from PIE root *em- "to take, distribute"). Related: Sumptuously; sumptuousness.

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