Etymology
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gold (n.)

"precious metal noted for its color, luster, malleability, and freedom from rust or tarnish," Old English gold, from Proto-Germanic *gulthan "gold" (source also of Old Saxon, Old Frisian, Old High German gold, German Gold, Middle Dutch gout, Dutch goud, Old Norse gull, Danish guld, Gothic gulþ), from PIE root *ghel- (2) "to shine," with derivatives denoting gold (the "bright" metal).

The root is the general Indo-European one for "gold," found in Germanic, Balto-Slavic (compare Old Church Slavonic zlato, Russian zoloto, "gold"), and Indo-Iranian. Finnish kulta is from German; Hungarian izlot is from Slavic. For Latin aurum see aureate. Greek khrysos probably is from Semitic.

From Homer on through Middle English, "red" often is given as a characteristic color of pure gold or objects made from it. This seems puzzling, but it might stem from an ancient practice of testing the purity of gold by heating it; in Middle English red gold was "pure gold" (c. 1200).

Nay, even more than this, the oftener gold is subjected to the action of fire, the more refined in quality it becomes; indeed, fire is one test of its goodness, as, when submitted to intense heat, gold ought to assume a similar colour, and turn red and igneous in appearance; a mode of testing which is known as "obrussa." [Pliny, "Natural History," 33.19]
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gold (adj.)

c. 1200, from gold (n.); compare golden. In reference to the color of the metal, it is recorded from c. 1400. Gold rush is attested from 1859, originally in an Australian context. Gold medal as first prize is from 1757. Gold record, a framed, gold phonograph record to commemorate a certain level of sales, is from 1948.

Joe Grady and Ed Hurst, WPEN disk jockey team, will be given a gold record by Mercury of the one-millionth copy of Frankie Lane's waxing of That's My Desire, January 10, for having done so much to plug the platter in these parts [Philadelphia]. [Billboard magazine, Jan. 10, 1948]
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gold-brick (n.)
"gold in the form of a brick," 1853, from gold (adj.) + brick (n.). Meaning "shirker" is from 1914, World War I armed forces slang, from earlier verb meaning "to swindle, cheat" (1902) from the old con game of selling spurious "gold" bricks (attested by 1881).
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guinea pig (n.)
rodent native to South America, 1660s. It does not come from Guinea and has nothing to do with the pig. Perhaps so called either because it was brought back to Britain aboard Guinea-men, ships that plied the triangle trade between England, Guinea, and South America [Barnhart, Klein], or from its resemblance to the young of the Guinea-hog "river pig" [OED], or from confusion of Guinea with the South American region of Guyana (but OED is against this). Pig probably for its grunting noises. In the extended sense of "one subjected to an experiment" it is first recorded 1920, because they were commonly used in medical experiments (by 1865).
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gold-digger (n.)
1816, "one who seeks gold in the ground or a stream bed," from gold (n.) + digger. As "woman who pursues men for their money," first recorded 1915.
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gold-mine (n.)
late 15c., "place where gold is dug out of the earth," from gold (n.) + mine (n.). Figurative use "anything productive of great wealth" is by 1882.
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