en- (2) Look up en- at Dictionary.com
word-forming element meaning "near, at, in, on, within," from Greek en "in," cognate with Latin in (see in), and thus with en- (1). Typically assimilated to em- before -p-, -b-, -m-, -l-, and -r-.
en- (1) Look up en- at Dictionary.com
word-forming element meaning "in; into," from French and Old French en-, from Latin in- "in, into" (see in- (2)). 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.