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.
"armorial bearings, coat of arms," late 13c., from Old French blason (12c.) "a shield, blazon," also "collar bone;" common Romanic (compare Spanish blason, Italian blasone, Portuguese brasao, Provençal blezo, the first two said to be French loan-words) but of uncertain origin. OED doubts, on grounds of sense, the connection proposed by 19c. French etymologists to Germanic words related to English blaze (n.1).