insulate (v.)

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.

noun of action from insulate (v.) in its various senses. From 1767 as "a blocking from electricity or heat" (by interposition of a non-conductor). Sense of "state or action of being detached from others" is from 1798. Literal meaning "act of making (land) into an island" is from 1784; that of "state of being an island" is from 1799. The concrete sense of "insulating material" is recorded by 1870.

1801, agent noun in Latin form from insulate (v.). In reference to the glass or earthenware devices to hold telegraph (later telephone) wires, from 1840s.