Etymology
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brain trust (n.)
"group of experts assembled to give advice on some matter," occasionally used since early 1900s, it became current in 1933, in reference to the intellectuals gathered by U.S. President Franklin D. Roosevelt as advisers; from brain (n.) + trust (n.).
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rank and file (n.)

1590s, in reference to the horizontal and vertical lines of soldiers marching in formation, from rank (n.) in the military sense of "number of soldiers drawn up in a line abreast" (1570s) + file (n.1). Thence generalized to "common soldiers" (1796) and "common people, general body" of any group (1860).

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Rh factor 
1942, from the first letters of rhesus; so called because the blood group, and its effects, were discovered in the blood of rhesus monkeys (1941).
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bobby sox (n.)

also bobby socks, 1943, from diminutive of bob (n.2) + sox. So called because they are "shortened" compared to knee-socks. Derivative bobby-soxer "adolescent girl," especially with reference to fans of popular crooners, first attested 1944.

Months ago colored bobby sox folded at the top were decreed, not by anyone or any group but, as usual, by a sudden mysterious and universal acceptance of the new idea. Now no teen-ager dares wear anything but pure white socks without a fold. [Life magazine, Dec. 11, 1944]
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West Indies 
Caribbean islands explored by Columbus, 1550s, reflecting the belief (or hope) that they were western outliers of the Indies of Asia. Related: West Indian, which is from 1580s in reference to the native inhabitants, 1650s in reference to European settlers there, and 1928 in reference to people of West Indian ancestry.
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ground zero (n.)
1946, originally with reference to atomic blasts. In reference to the Sept. 11, 2001, terrorist attack on New York, it was in use by Sept. 13.
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West Bank 
in reference to the former Jordanian territory west of the River Jordan, 1967.
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home rule (n.)
1860, originally in reference to Ireland, from home (n.) + rule (n.).
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black sheep (n.)
by 1822 in figurative sense of "member of some group guilty of offensive conduct and unlike the other members," supposedly because a real black sheep had wool that could not be dyed and thus was worth less. But one black sheep in a flock was considered good luck by shepherds in Sussex, Somerset, Kent, Derbyshire. First known publication of Baa Baa Black Sheep nursery rhyme is in "Tommy Thumb's Pretty Song Book" (c. 1744).
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sub voce 
Latin, literally "under the word or heading." A common dictionary reference, usually abbreviated s.v.
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