Etymology
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horsepower (n.)
also horse-power, unit for measurement of the rate at which a motor works, 1805, from horse (n.) + power (n.); established by Watt as the power needed to lift 33,000 pounds one foot in one minute, which actually is about 1.5 times the power of a strong horse. Much abused in 19c. technical writing as "very fallacious," "shockingly unscientific," etc.
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soup (v.)
"increase the horsepower of an engine," 1921, probably from soup (n.) in slang sense of "narcotic injected into horses to make them run faster" (1911), influenced by supercharge (v.).
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