Etymology
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Prussia (n.)

region in northeastern Germany, late 14c., Prusse (late 13c. as a surname), from Medieval Latin Borussi, Prusi, Latinized forms of the native name of the Lithuanian people who lived in the bend of the Baltic before being conquered 12c. and exterminated by (mostly) German crusaders who replaced them as the inhabitants.

Perhaps from Slavic *Po-Rus "(Land) Near the Rusi" (i.e. Russians; compare Pomerania). The German duchy of Prussia after the 17c. union with the Mark of Brandenberg became the core of the Prussian monarchy and later the chief state in the German Empire. The center of power shifted to Berlin after the union, and the old core of the state came to be known as East Prussia.

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Borussia 
alternative form of Prussia (q.v.).
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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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Prussian 

1550s (n.), "native or inhabitant of Prussia;" 1560s (adj.), "of or pertaining to Prussia;" from Prussia + -an. In reference to the language of the earlier inhabitants of (East) Prussia, which was closely related to Lithuanian, by 1888. It was spoken between the lower Vistula and the Niemen and was extinct by the end of 17c.  Prussian blue pigment (1724) came to English from French bleu de Prusse, so called for being discovered in Berlin, the Prussian capital.

All in all, it seems that Prussian blue was synthesised for the first time around 1706 by the Swiss immigrant Johann Jacob Diesbach in Berlin. [Jens Bartoll and Bärbel Jackisch, "Prussian Blue: A Chronology of the Early Years," in Zeitschrift für Kunsttechnologie und Konservierung 24, No. 1, 2010]

Early German sources refer to it as Preußisches Ultra-Marin and berliner blau. Prussic acid (1790), is from French acide prussique, so called in reference to Prussian blue pigment, to which it is chemically related.

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Pomeranian (n.)

type of small Spitz-type dog, 1760, from Pomerania, former province of Prussia on the south coast of the Baltic Sea.

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hegemonist (n.)
"one who advocates a political policy of hegemony," 1898 (in reference to Prussia in Germany); see hegemony + -ist.
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Iron Cross 
from German eiserne kreuz, instituted 1813 by Frederick Wilhelm III of Prussia, originally for distinguished military service in the wars against Napoleon.
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spruce (adj.)
"neat, smart in dress and appearance, dapper, brisk," 1580s, from spruce leather (mid-15c.; see spruce (n.)), a type of leather imported from Prussia in the 1400s and 1500s which was used in England to make a popular style of jerkin that was considered smart-looking.
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Pomerania (n.)

former duchy and province of Prussia on the Baltic coast of modern Poland (German Pommern, Polish Pomorze), Medieval Latin, from Pomerani, name of a Slavic tribe there, from Polish po morze "by the sea."

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homburg (n.)
type of soft felt hat with a curled brim and a dented crown, 1894, from Homburg, resort town in Prussia, where it was first made. Introduced to England by Edward VII.
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