word-forming element meaning "in; into," from French and Old French en-, from Latin in- "in, into" (from PIE root *en "in"). Typically assimilated before -p-, -b-, -m-, -l-, and -r-. Latin in- became en- in French, Spanish, Portuguese, but remained in- in Italian.
Also used with native and imported elements to form verbs from nouns and adjectives, with a sense "put in or on" (encircle), also "cause to be, make into" (endear), and used as an intensive (enclose). Spelling variants in French that were brought over into Middle English account for parallels such as ensure/insure, and most en- words in English had at one time or another a variant in in-, and vice versa.
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.