Isle
Middle English ile, from Old French, from Latin insula . The spelling with s (also in 15th-century French) is influenced by Latin.
wiktionary
From Middle English ile, yle (with s added, similar to English island), borrowed from Old French ille, idle, isle, from Latin insula. Not related to island. Doublet of insula.
isle (plural isles)
etymonline
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.
