Origin and history of island

the usual Old English word for "river, running water" (still in use in Lancashire, according to OED), from Proto-Germanic *ahwo- (source also of Old Frisian a, Old Saxon aha, Old High German aha, German ahe-, Old Dutch aha, Old Norse "water"), from PIE root *ekweh- "water" (see aqua-). "The standard word in place-names for river denoting a watercourse of greater size than a broc or a burna" [Cambridge Dictionary of English Place-Names].

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.