Etymology
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Slavic (adj.)
1813; see Slav + -ic. Earlier in same sense was Slavonic (1640s), from Slavonia, a region of Croatia; Slavonian (1570s). As a noun in reference to a language group from 1812.
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regroup (v.)

also re-group, "to group again, form anew into a group," 1838, from re- "again" + group (v.). Related: Regrouped; regrouping; regroupment.

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subgroup (n.)
also sub-group, 1825, from sub- + group (n.).
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Pentothal 
trademark name of an anaesthetic and hypnotic, 1935, refashioning of Thiopental, from pento-, in reference to the methylbutyl five-carbon group (from PIE root *penkwe- "five") + first two letters of thiobarbiturate + chemical product suffix -ol.
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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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grouping (n.)
"act, process, or result of arranging in a group," 1748, verbal noun from group (v.).
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septenary (adj.)

"consisting of or relating to seven," c. 1600, from Latin septenarius "consisting of seven," from septeni (plural) "seven apiece, by sevens," from septem "seven" (see seven). Especially in reference to the week. As a noun, "group or set of seven," from 1590s; as "period of seven years," 1570s.

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old-school (adj.)

in reference to a group of people noted for conservative views or principles on some professional or political matter, 1806, from the noun phrase, "party belonging to a former time or having the characteristics, manner, and opinions of a bygone age" (1749); see old + school (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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