Etymology
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fin (n.)
Old English finn "fin," from Proto-Germanic *finno (source also of Middle Low German vinne, Dutch vin), perhaps from Latin pinna "feather, wing" (see pin (n.)); or, less likely, from Latin spina "thorn, spine" (see spine).

U.S. underworld slang sense of "$5 bill" is 1925, from Yiddish finif "five," from German fünf (from PIE root *penkwe- "five") and thus unrelated. The same word had been used in England in 1868 to mean "five pound note" (earlier finnip, 1839).
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fin de siecle (adj.)

1890, from French fin de siècle "end of century," phrase used as an adjective. At the time it meant "modern;" now it means "from the 1890s." "App. first in title of a comedy, Paris fin de siècle, produced at the Gymnase, Feb. 1890" [Weekley]. French siècle "century, age" is from Latin saeculum "age, span of time, generation" (see secular).

No proof is needed of the extreme silliness of the term. Only the brain of a child or of a savage could form the clumsy idea that the century is a kind of living being, born like a beast or a man, passing through all the stages of existence, gradually ageing and declining after blooming childhood, joyous youth, and vigorous maturity, to die with the expiration of the hundredth year, after being afflicted in its last decade with all the infirmities of mournful senility. [Max Nordau, "Degeneration," English translation, 1895]
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finned (adj.)
mid-14c., adjective in past participle form from fin.
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pinniped (n.)

"a fin-footed mammal," one having feet like fins or flippers, especially of the group of fin-footed aquatic carnivorous quadruped mammals that includes seals, sea-lions, and walruses, 1842, from Modern Latin Pinnipedia, suborder of aquatic carnivorous mammals (seals and walruses), literally "having feet as fins," from Latin pinna in its secondary sense "fin" (see pin (n.)) + pes, genitive pedis "foot" (from PIE root *ped- "foot").

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fingering (n.2)
"thick, loose woolen yarn," 1680s, from fingram, from French fin grain, literally "fine grain."
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finesse (n.)

1520s, "fineness" (obsolete); 1530s, "artifice, delicate stratagem," from French finesse "fineness, subtlety," from Old French fin "subtle, delicate" (see fine (adj.)).

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sailfish (n.)

also sail-fish, "fish with a long or large dorsal fin," 1590s, from sail (n.) + fish (n.).

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flipper (n.)
limb used to swim with, 1822, agent noun from flip (v.). Sense of "rubber fin for underwater swimming" is from 1945. Slang meaning "the hand" dates from 1836. Related: Flippers.
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branchial (adj.)
"of or pertaining to gills," 1774, from Modern Latin branchialis, from Latin branchiae "gills," from Greek brankhia "gills," plural of brankhion "fin." Related: Branchiate.
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