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.
mid-14c., "net to catch fish" (implied in trammeller "one who fishes with a trammel net"), from Old French tramail "fine-gauged fishnet" (13c.), from Late Latin tremaculum, perhaps meaning "a net made from three layers of meshes," from Latin tri- "three" (see tri-) + macula "a mesh" (see mail (n.2)). Meaning "anything that hinders" is from 1650s, originally "a hobble for a horse" (c. 1500). Italian tramaglio, Spanish trasmallo are French loan-words.