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ultra vires 

Latin, literally "beyond powers," from ultra "beyond" (see ultra-) + vires "strength, force, vigor, power," plural of vis (see vim). Usually "beyond the legal or constitutional power of a court, etc."

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Uncle Tom (n.)

"servile black man," 1922, somewhat inaccurately in reference to the humble, pious, but strong-willed main character in Harriet Beecher Stowe's novel, "Uncle Tom's Cabin" (1852). The image implied in the insult perhaps is more traceable to the late 19c. minstel show versions of the story, which reached a far wider audience than the book.

I don't recall anyone in the 1920s using the term 'Uncle Tom' as an epithet. But what's amazing is how fast it caught on (in the 1930s). Black scholars picked up (the term) and just started throwing it at each other. [Ernest Allen, quoted in Hamilton, Kendra, "The Strange Career of Uncle Tom," Black Issues in Higher Education, June 2002]

As a verb, attested from 1937.

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Underground Railroad (n.)

"network of U.S. anti-slavery activists helping runaways elude capture," attested from 1847, but said to date from 1831 and to have been coined in jest by bewildered trackers after their slaves vanished without a trace. Originally mostly the term for escape networks in the (then) western states of the U.S.

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United Nations 

1942, "the Allied nations at war with the Axis powers;" the international body (officially the United Nations Organization) was chartered in 1945.

Such negotiation as may occur in New York is not conducted within the walls of the tall building by the East River: it is carried out elsewhere, in accordance with those principles of courtesy, confidence and discretion which must for ever remain the only principles conducive to the peaceful settlement of disputes. [Harold Nicholson, "The Evolution of Diplomatic Method," 1954]
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united states (n.)

attested from 1617, originally with reference to Holland; the North American confederation first so called in 1776. United Provinces were the seven northern provinces of the Netherlands, allied from 1579, later developing into the kingdom of Holland.

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upside down (adv.)

late 15c., earlier upsadoun (late 14c.), up so down (c. 1300); the so perhaps meaning "as if." As an adjective from 1866.

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