Etymology
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forum (n.)
mid-15c., "place of assembly in ancient Rome," from Latin forum "marketplace, open space, public place," apparently akin to foris, foras "out of doors, outside," from PIE root *dhwer- "door, doorway." Sense of "assembly, place for public discussion" first recorded 1680s.
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forensic (adj.)
"pertaining to or suitable for courts of law," 1650s, with -ic + stem of Latin forensis "of a forum, place of assembly," related to forum "public place" (see forum). Later used especially in sense of "pertaining to legal trials," as in forensic medicine (1845). Related: Forensical (1580s).
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agora (n.)

1590s, "open assembly place, chief public square and marketplace of a town; popular political assembly held in such a place," from Greek agora "an assembly of the People" (as opposed to a council of Chiefs); "the place of assembly; a marketplace" (the typical spot for such an assembly), from ageirein "to assemble" (from PIE root *ger- "to gather").

The Greek word also could mean "public speaking," and "things to be sold." For sense, compare Roman forum.

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*dhwer- 

Proto-Indo-European root meaning "door, doorway." The base form is frequently in dual or plural, leading to speculation that houses of the original Indo-Europeans had doors with two swinging halves.

It forms all or part of: afforest; deforest; door; faubourg; foreclose; foreign; forensic; forest; forfeit; forum; hors d'oeuvre; thyroid.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit duárah "door, gate;" Old Persian duvara- "door;" Lithuanian dùrys (plural); Greek thyra "door;" Latin foris "out-of-doors, outside;" Gaulish doro "mouth;" Old Prussian dwaris "gate;" Russian dver' "a door;" Old English dor, German Tür "door," Gothic dauro "gate."

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quintain (n.)

"target for tilting and jousting practice," c. 1400 (in Anglo-Latin from mid-13c.), from Old French quintaine or directly from Medieval Latin quintana; perhaps from Latin quintana "of the fifth" (from PIE root *penkwe- "five"), which as a noun meant "the business part of a camp," a street where the market and forum were located, on the supposition that this was where military exercises were done [OED].

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manes (pl.)

in Roman religion, "spirits of the dead considered as tutelary divinities of their families," from Latin manes "departed spirit, ghost, shade of the dead, deified spirits of the underworld," usually said to be related to Latin manus "good," thus properly "the good gods," a euphemistic word. De Vaan cites cognates Old Irish maith, Welsh mad, Breton mat "good." The ultimate etymology is uncertain (compare mature).

Three times a year a pit called the mundus was officially opened in the comitium of the Roman Forum, to permit the manes to come forth. The manes were also honored at certain festivals, as the Parentalia and Feralia; oblations were made to them, and the flame maintained on the altar of the household was a homage to them. [In this sense often written with a capital.] [Century Dictionary]
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rostrum (n.)

"pulpit or platform from which a speaker addresses an audience," 1540s, originally in an ancient Roman context, from Latin rostrum, the name of the platform stand for public speakers in the Forum in ancient Rome. It was decorated with the beaks of ships taken in the first naval victory of the Roman republic, over Antium, in 338 B.C.E., and the Latin word's older sense is "end of a ship's prow," literally "beak, muzzle, snout," originally "means of gnawing," an instrument noun from rodere "to gnaw" (see rodent).

The beaks were an ancient form of ram, a beam spiked with pointed iron for the purpose of sinking other vessels. For the form, compare claustrum "lock, bar," from claudere "to shut." The extended sense, in reference to any platform for public speaking, is attested by 1766.

The classical plural is rostra, though in English this is more common in the original "ship's beak" sense and -rums often is used in the secondary sense.

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forest (n.)
late 13c., "extensive tree-covered district," especially one set aside for royal hunting and under the protection of the king, from Old French forest "forest, wood, woodland" (Modern French forêt), probably ultimately from Late Latin/Medieval Latin forestem silvam "the outside woods," a term from the Capitularies of Charlemagne denoting "the royal forest." This word comes to Medieval Latin, perhaps via a Germanic source akin to Old High German forst, from Latin foris "outside" (see foreign). If so, the sense is "beyond the park," the park (Latin parcus; see park (n.)) being the main or central fenced woodland.

Another theory traces it through Medieval Latin forestis, originally "forest preserve, game preserve," from Latin forum in legal sense "court, judgment;" in other words "land subject to a ban" [Buck]. Replaced Old English wudu (see wood (n.)). Spanish and Portuguese floresta have been influenced by flor "flower."
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