Etymology
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fir (n.)
late 14c., from Old Norse fyri- "fir" or Old Danish fyr, both from Proto-Germanic *furkhon (source also of Old High German foraha, German Föhre "fir"), from PIE root *perkwu-, originally meaning "oak," also "oak forest," but never "wood" (source also of Sanskrit paraktah "the holy fig tree," Hindi pargai "the evergreen oak," Latin quercus "oak," Lombardic fereha "a kind of oak"). Old English had a cognate form in furhwudu "pine wood" (only in glosses, for Latin pinus), but the modern English word is more likely from Scandinavian and in Middle English fyrre glosses Latin abies "fir," which is of obscure origin.

According to Indo-Europeanists Gamkrelidze and Ivanov, "The semantics of the term clearly points to a connection between 'oak' and mountainous regions, which is the basis for the ancient European term applied to forested mountains" (such as Gothic fairgunni "mountainous region," Old English firgen "mountain forest," Middle High German Virgunt "mountain forest; Sudetes"). In the period 3300 B.C.E. to 400 B.C.E., conifers and birches gradually displaced oaks in northern European forests. "Hence it is no surprise that in the early history of the Germanic languages the ancient term for mountain oak and oak forest shifts to denote conifers and coniferous forests." [Thomas V. Gamkrelidze, Vjaceslav V. Ivanov, "Indo-European and the Indo-Europeans," Berlin, 1994]
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Quercus (n.)

tree genus, Latin quercus "oak," from PIE *kwerkwu-, assimilated form of *perkwu- "oak" (see fir). Related: Quercine (adj.).

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Hercynian (adj.)
1580s, a classical term of vague application designating the forest-covered mountains of ancient Germany (especially das Harzgebirge), from Latin hercynia (silva) "Hercynian (forest)," related to Greek herkynios (drymos), probably from Old Celtic *perkunya, from PIE *perq(o)- "oak, oak forest, wooded mountain" (see fir). As a term in geology from 1880.
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pine (n.)

"coniferous tree, tree of the genus Pinus," Old English pin (in compounds), from Old French pin and directly from Latin pinus "pine, pine-tree, fir-tree," which is perhaps from a PIE *pi-nu-, from root *peie- "to be fat, swell" (see fat (adj.)).

If so, the tree's name would be a reference to its sap or pitch. Compare Sanskrit pituh "juice, sap, resin," pitudaruh "pine tree," Greek pitys "pine tree." Also see pitch (n.1). The native Old English word was furh (see fir). Pine-top "cheap illicit whiskey," is attested by 1858, Southern U.S. slang.

Most of us have wished vaguely & vainly at times that they knew a fir from a pine. As the Scotch fir is not a fir strictly speaking, but a pine, & as we shall continue to ignore this fact, it is plain that the matter concerns the botanist more than the man in the street. [Fowler]
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Firbolgs (n.)
1797, ancient supernatural people of Ireland (enemies of the Dannans); according to OED perhaps from Old Irish fir, plural of fear "man" + bolg, genitive plural of bolg "bag, belly" (from PIE *bhelgh- "to swell," extended form of root *bhel- (2) "to blow, swell"). Or the second element may be cognate with the Gaulish tribal name Belgae. Related: Firbolgian.
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spruce (n.)
1660s, "evergreen tree, fir," from spruse (adj.) "made of spruce wood" (early 15c.), literally "from Prussia," from Spruce, Sprws (late 14c.), unexplained alterations of Pruce "Prussia," from an Old French form of Prussia.

Spruce seems to have been a generic term for commodities brought to England by Hanseatic merchants (especially beer, boards and wooden chests, and leather), and the tree thus was believed to be particular to Prussia, which for a time was figurative in England as a land of luxuries. Compare spruce (adj.).

As a distinct species of evergreen tree from 1731. Nearly all pines have long, soft needles growing in groups of two (Scotch) to five (white); spruce and fir needles grow singly. Spruce needles are squarish and sharp; fir needles are short and flat. Cones of the fir stand upright; cones of a spruce hang before falling.
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deal (n.2)

"plank or board," especially of fir or pine, late 14c., dele, from Low German (compare Middle Low German dele), from Proto-Germanic *theljon." From late 13c. in surnames. An Old English derivative was þelu"hewn wood, board, flooring."

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conifer (n.)

"a plant producing cones, a plant of the order Coniferae" (which includes pine, fir, and cypress trees), 1847, from Latin conifer "cone-bearing, bearing conical fruit," from conus "cone" (see cone) + ferre "to bear, carry" (from PIE root *bher- (1) "to carry").

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turpentine (n.)
early 14c., "semi-liquid resin of the terebinth tree," terbentyn, from Old French terebinte "turpentine" (13c.), from Latin terebintha resina "resin of the terebinth tree," from Greek rhetine terebinthe, from fem. of terebinthos (see terebinth). By 16c. applied generally to resins from fir trees.
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hemlock (n.)
poisonous plant native to Europe, transplanted to North America, Old English (Kentish) hemlic, earlier hymlice, hymblice, name of a poisonous plant; of unknown origin. Liberman suggests from root hem- "poison," perhaps with the plant name suffix -ling or -ig. As the name of the poison derived from the plant, c. 1600. The North American fir tree so called by 1670s in New England, from resemblance of the position and tenuity of its leaves to those of the plant.
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