Etymology
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group (v.)
"form into a group or groups," 1718 (transitive), 1801 (intransitive), from group (n.). Related: Grouped; grouping.
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reference (v.)

1620s, "to assign;" as "to provide a reference to, find by reference," from 1837 (implied in referenced); from reference (n.). Related: Referencing.

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group (n.)

1690s, originally an art criticism term, "assemblage of figures or objects forming a harmonious whole in a painting or design," from French groupe "cluster, group" (17c.), from Italian gruppo "group, knot," which probably is, with Spanish grupo, from a Germanic source, from Proto-Germanic *kruppaz "round mass, lump," part of the general group of Germanic kr- words with the sense "rounded mass" (such as crop (n.).

Extended to "any assemblage, a number of individuals related in some way" by 1736. Meaning "pop music combo" is from 1958. As it was borrowed after the Great Vowel Shift in English, the pronunciation of the -ou- follows French rather than English models.

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reference (n.)

1580s, "act of referring" (some matter, to someone for consideration), from refer + -ance, or else from French référence, from Medieval Latin *referentia, from Latin referentem (nominative referens), present participle of referre.

Meaning "direction to a book or passage" where certain information may be found is recorded from 1610s. By 1837 as "one who or that which may be referred to." The meaning "testimonial" is from 1895. Reference book , a dictionary, encyclopedia, or similar book intended to be consulted as occasion requires, dates from 1808; reference library is by 1834. Phrase in reference to is attested from 1590s. "By slipshod extension, the word is often now made to mean a person to whom r[eference] is permitted as a witness to character, & even a written testimonial" [Fowler, 1926]. The earlier word for "one who gives characters for people seeking employment" was referee (1862) but this word had a bad savor, of literate accomplices of professional beggars and thieves.

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age-group (n.)
1876, originally a term in the science of demographics, from age (n.) + group (n.).
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cross-reference (n.)

also crossreference, "a reference in a book to another part of it," 1834, from cross- + reference (n.). As a verb, "refer to by cross-reference," attested by 1851. Related: Cross-referenced; cross-referencing.

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ennead (n.)
"group of nine things," 1650s, from Greek enneas (genitive enneados) "group of nine," from ennea "nine" (see nine). Especially in reference to the divisions of Porphyry's collection of the neo-Platonic doctrines of Plotinus. Related: enneadic.
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Shetland 
group of islands north of Scotland, from Old Norse Hjaltland; in reference to a type of pony, 1801; as a breed of sheep, 1794.
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tranquilizer (n.)
1800, "that which tranquilizes;" from 1824 as "a sedative" (first reference is to ground ivy), agent noun from tranquilize; in reference to one of a large group of anti-anxiety drugs, it is recorded by 1956.
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bloc (n.)
1903, in reference to alliances in Continental politics, from French bloc "group, block," from Old French bloc "piece of wood" (see block (n.1)).
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