Etymology
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isle (n.)

late 13c., ile, from Old French ile, earlier isle, from Latin insula "island," a word of uncertain origin.

Perhaps (as the Ancients guessed) from in salo "(that which is) in the (salty) sea," from ablative of salum "the open sea," related to sal "salt" (see salt (n.)). De Vaan finds this "theoretically possible as far as the phonetics go, but being 'in the sea' is not a very precise description of what an island is; furthermore, the Indo-Europeans seem to have indicated with 'island' mainly 'river islands.' ... Since no other etymology is obvious, it may well be a loanword from an unknown language." He proposes the same lost word as the source of Old Irish inis, Welsh ynys "island" and Greek nēsos "island." The -s- was restored first in French, then in English in the late 1500s.

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enisle (v.)
c. 1600, from en- (1) "in, into" + isle (n.).
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islet (n.)

1530s, from French islette (Modern French îlette), diminutive of isle (see isle).

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insula (n.)
Latin, literally "an island" (also, in ancient Rome, "a block of buildings"); see isle. In anatomical use, the notion is "detached or standing out by itself."
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insulin (n.)
1922 (earlier insuline, 1914), coined in English from Latin insula "island" (see isle and compare insula); so called because the hormone is secreted by the islets of Langerhans in the pancreas. Insuline was coined independently in French in 1909.
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peninsula (n.)

"piece of land almost surrounded by water but connected with a mainland by a neck or isthmus," 1530s, from Latin paeninsula "a peninsula," literally "almost an island," from pæne "nearly, almost, practically," which is of uncertain origin, + insula "island" (see isle). In 16c. sometimes Englished as demie island.

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insulate (v.)
1530s, "make into an island," from Late Latin insulatus "made like an island," from insula "island" (see isle). Sense of "place in an isolated situation, cause (someone or something) to be detached from surroundings" is from 1785. Electrical/chemical sense of "block from electricity or heat" (by interposition of a non-conductor) is from 1742. Related: Insulated; insulating.
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insular (adj.)
1610s, "of or pertaining to an island," from Late Latin insularis "of or belonging to an island," from Latin insula "island" (see isle). Metaphoric sense "narrow, prejudiced" is from 1775, from notion of being isolated and cut off from intercourse with other nations or people (an image that naturally suggested itself in Great Britain). The earlier adjective in the literal sense was insulan (mid-15c.), from Latin insulanus.
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Chersonese 

peninsula south of Thrace, from Greek khersonesos "peninsula," from khersos "dry land" + nēsos "island," also "(flooded) land near a river, alluvial land," which is of uncertain origin; traditionally from PIE root sna- "to swim," but this is now generally rejected. "As words for 'island' differ from language to language, [nēsos] is probably an Aegean loan (note that Lat. insula is also of unclear origin)" [Beekes]. Compare isle.

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isolated (adj.)

"standing detached from others of its kind," 1740, a rendering into English of French isolé "isolated" (17c.), from Italian isolato, from Latin insulatus "made into an island," from insula "island" (see isle (n.)). English at first used the French word (isole, also isole'd, c. 1750), then after isolate (v.) became an English word, isolated became its past participle.

[F]or I think it very natural to suppose, that all the People of this isolated Country, (I ask Pardon for a foreign Word) should have one Language ... ["An Irregular Dissertation Occasioned by the Reading of Father Du Halde's Description of China," 1740]
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