Etymology
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gang (v.)

1856, from gang (n.). Related: Ganged; ganging. To gang up (on) is attested by 1919.

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pentagon (n.)

1560s, "plane figure with five angles and five sides," from French pentagone (13c.) or directly from Late Latin pentagonum "pentagon," from Greek pentagōnon, a noun use of the neuter of the adjective pentagōnos "five-angled," from pente "five" (from PIE root *penkwe- "five") + gōnia "angle, corner" (from PIE root *genu- (1) "knee; angle"). The U.S. military headquarters known as the Pentagon was completed in 1942, and so called for its shape; used allusively for "U.S. military leadership" from 1945; Pentagonese "U.S. official military jargon" is by 1951. Related: Pentagonal.

In nature, pentagonal symmetry is rare in inanimate forms. Packed soap bubbles seem to strive for it but never quite succeed, and there are no mineral crystals with true pentagonal structures. But pentagonal geometry is basic to many living things, from roses and forget-me-nots to sea urchins and starfish. [Robert Bringhurst, "The Elements of Typographic Style," 1992]
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gang (n.)

from Old English gang "a going, journey, way, passage," and Old Norse gangr "a group of men, a set," both from Proto-Germanic *gangaz (source also of Old Saxon, Old Frisian, Danish, Dutch, Old High German, German gang, Old Norse gangr, Gothic gagg "act of going"), of uncertain origin, perhaps from PIE root *ghengh- "to step" (source also of Sanskrit jangha "shank," Avestan zanga- "ankle," Lithuanian žengiu "I stride"). Not considered to be related to go.

The sense evolution is probably via meaning "a set of articles that usually are taken together in going" (mid-14c.), especially a set of tools used on the same job. By 1620s this had been extended in nautical speech to mean "a company of workmen," and by 1630s the word was being used, with disapproving overtones, for "any band of persons traveling together," then "a criminal gang or company" (gang of thieves, gang of roughs, etc.). By 1855 gang was being used in the sense "group of criminal or mischievous boys in a city." In American English, especially of slaves working on plantations (1724). Also formerly used of animal herds or flocks (17c.-19c.). Gangway preserves the original sense of the word, as does gang-plank.

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press-gang (n.)

"detachment under command of an officer empowered to press men into public service," 1690s, from press (v.2) + gang (n.).

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chain-gang (n.)

"a number of slaves or convicts chained together outdoors doing labor or during transit," 1816, from chain (n.) + gang (n.). 

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Stern gang (n.)
militant Zionist terrorist organization (officially Lohame Herut Yisra'el "Fighters for the Freedom of Israel") founded 1940 by Avram Stern (1907-1942).
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gang-bang (n.)
1953, "group sex" (especially many men on one woman or girl, regardless of consent), from gang + bang (v.) in its slang, "perform sexual intercourse" sense. Earlier was gang-shag (1927). Sense of "participate in a street gang" is by 1968. Related: Gang-banger; gang-banging.
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gang-plank (n.)
also gangplank, 1842, American English, from gang in its nautical sense of "a path for walking, passage" (see gangway) + plank. Replacing earlier gang-board.
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Gang of Four 
1976, translating Chinese sirenbang, the nickname given to the four leaders of the Cultural Revolution who took the fall in Communist China after the death of Mao.
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gangster (n.)
"member of a criminal gang," 1896, American English, from gang (n.) in its criminal sense + -ster. Related: Gangsterism (1918).
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