Online Etymology Dictionary

en- (1): assimilated to -p-, -b-, -m-, -l-, -r-, word-forming element meaning "in, into," from French and Old French en-, from Latin in- "in, into" (see in- (2)).

Also used with native elements to form verbs from nouns and adjectives, "put in or on" (encircle), also "cause to be, make" (endear), and used as an intensive (enclose). Spelling variants in French that were brought over into Middle English account for parallels such as assure/ensure/insure.
en- (2): assimilated to -p-, -b-, -m-, -l-, -r-, word-forming element meaning "near, at in, on, within," from Greek en "in," cognate with Latin in (see in), and with en- (1).
enable (v.): early 15c., "to make fit;" mid-15c., "to make able to," from en- (1) "make, put in" + able. Related: Enabled; enabling.
enabler (n.): 1610s, agent noun from enable.
enact (v.): early 15c., from en- (1) "make, put in" + act. Related: Enacted; enacting.
enactment (n.): 1817, from enact + -ment.
enamel (v.): early 14c., from Anglo-French enamailler (early 14c.), from en- "in" (see en- (1)) + amailler "to enamel," variant of Old French esmailler, from esmal "enamel," from Frankish *smalt, from Proto-Germanic *smaltjan "to smelt" (see smelt (v.)). Related: Enameled; enameling.
enamel (n.): early 15c., from enamel (v.).
enamor (v.): c.1300, from Old French enamorer "to fall in love with; to inspire love (12c., Modern French enamourer), from en-, causative prefix (see en- (1)), + amour "love," from amare "to love" (see Amy). An equivalent formation to Provençal, Spanish, Portuguese enamorar, Italian innamorare.
enamored (adj.): 1630s, past participle adjective from enamor.
enamour (v.): chiefly British English form of enamor; for spelling, see -or. Related: Enamoured.
encamp (v.): 1560s, from en- (1) "make, put in" + camp (n.). Related: Encamped; encamping.
encampment (n.): 1590s, from encamp + -ment.
encapsulate (v.): 1874, from en- (1) "make, put in" + capsulate (see capsule). Related: Encapsulated; encapsulating.
encapsulation (n.): 1860, from encapsulate + -ion.
encase (v.): 1630s, from en- (1) "make, put in" + case (n.2). Related: Encased; encasing.
encaustic: c.1600 (n.); 1650s (adj.), from Greek enkaustikos, from enkaiein "to burn in" from en (see en- (2)) + kaiein "to burn" (see caustic).
enceinte (adj.): c.1600, insente, from French enceinte "pregnant" (12c.), from Late Latin incincta (source of Italian incinta), usually explained as "ungirt," from Latin in-, privative prefix (see in- (1)), + cincta, fem. of cinctus, past participle of cingere "to gird" (see cinch). Modern form is 18c., perhaps a reborrowing from French.
encephalitis (n.): 1843, from medical Latin encephalon (from Greek enkephalos "brain," literally "within the head," from en "in" + kephale "head;" see cephalo-) + -itis.
enchant (v.): late 14c., literal and figurative, from Old French enchanter "bewitch, charm, cast a spell" (12c.), from Latin incantare (see enchantment). Or perhaps a back-formation from enchantment. Related: Enchanting; enchantingly. Enchanted in weakened sense of "delighted" is from 1590s.
enchanter (n.): late 13c., agent noun from enchant or from Old French enchanteor.
enchantment (n.): late 13c., from Old French encantement, from enchanter "bewitch, charm," from Latin incantare, literally "enchant, cast a (magic) spell upon," from in- "upon, into" (see in- (2)) + cantare "to sing" (see chant (v.)). Figurative sense of "alluring" is from 1670s. Cf. Old English galdor "song," also "spell, enchantment," from galan "to sing," source of the second element in nightingale.
enchantress (n.): late 14c., "witch," from enchanter + -ess. Meaning "charming woman" is from 1713.
enchilada (n.): 1887, American English, from Mexican Spanish enchilada, fem. past participle of enchilar "season with chili," from en- "in" + chile "chili" (see chili).
enchiridion (n.): 1540s, "a handbook," from Late Latin, from Greek enkheiridion, neuter of enkheiridios "that which is held in the hand," from en "in" (see en- (2)) + kheir "hand" (see chiro-) + diminutive suffix -idion.
encircle (v.): c.1400, from en- (1) "make, put in" + circle. Related: Encircled; encircling; encirclement.
enclave (n.): 1868, from French enclave, from Old French enclaver "enclose, comprise, include" (13c.), from Late Latin inclavare "shut in, lock up," from Latin in- "in" (see in- (2)) + clavis "key" (see slot (n.2)). Enclaved "surrounded by land owned by another" is attested in English from mid-15c., from Middle French enclaver.
enclitic: 1650s (adj.); 1660s (n.), from Late Latin encliticus, from Greek enklitikos "throwing its accent back," literally "leaning on," from verbal adjectival stem of enklinein "to bend, lean on," from en (see en- (2)) + klinein "to lean" (see lean (v.)).
enclose (v.): early 14c., from en- (1) + close, and partially from Old French enclos, past participle of enclore.

Specific sense of "to fence in waste or common ground" for the purpose of cultivation or to give it to private owners, is from c.1500. Meaning "place a document with a letter for transmission" is from 1707. Related: Enclosed; enclosing.
enclosure (n.): mid-15c., "action of enclosing," from enclose + -ure. Meaning "that which is enclosed" is from 1550s.
encode (v.): 1919, from en- (1) "make, put in" + code. Computing sense is from 1955, usually shortened colloquially to code. Related: Encoded; encoding.
encomiast (n.): c.1600, from Greek enkomiastes "one who praises," from enkomiazein, from enkomion (see encomium). Related: Encomiastic (1590s).
encomienda (n.): 1810, from Spanish, "commission," from encomendar "to commit, charge." Estate granted to a Spaniard in America with powers to tax the Indians.
encomium (n.): 1580s, from Late Latin encomium, from Greek enkomion (epos) "laudatory (ode), eulogy," from en "in" (see en- (2)) + komos "banquet, procession, merrymaking" (see comedy).
encompass (v.): 1550s, from en- (1) "make, put in" + compass. Related: Encompassed; encompasses; encompassing.
encore (interj.): 1712, from French encore "still, yet, again" (12c.), generally explained as being from Vulgar Latin phrase *hinc ad horam "from then to this hour" (Italian ancora "again, still, yet" is said to be a French loan-word).
Whenever any Gentlemen are particularly pleased with a Song, at their crying out Encore ... the Performer is so obliging as to sing it over again. [Steele, "Spectator" No. 314, 1712]

There appears to be no evidence that either the Fr. or It. word was ever similarly used in its native country. The corresponding word both in Fr. and It. is bis; in It. da capo was formerly used. [OED]
As a noun, from 1763; as a verb, from 1748.
encounter (n.): c.1300, "meeting of adversaries, confrontation," from Old French encontre "meeting; fight; opportunity," noun use of preposition/adverb encontre "against, counter to" from Late Latin incontra "in front of," from Latin in- "in" (see in- (2)) + contra "against" (see contra). Modern use of the word in psychology is from 1967, from the work of U.S. psychologist Carl Rogers (1902-1987). Encounter group attested from 1967.
encounter (v.): c.1300, "to meet as an adversary," from Old French encontrer "confront," from encontre (see encounter (n.). Weakened sense of "casually meet" first recorded in English early 16c. Related: Encountered; encountering.
encourage (v.): early 15c., from Old French encoragier "make strong, hearten," from en- "make, put in" (see en- (1)) + corage (see courage). Related: Encouraged; encouraging.
encouragement (n.): 1560s, from encourage + -ment, or from Middle French encoragement.
As a general rule, Providence seldom vouchsafes to mortals any more than just that degree of encouragement which suffices to keep them at a reasonably full exertion of their powers. [Hawthorne, "House of Seven Gables"]
encroach (v.): early 14c., "acquire, get," from Old French encrochier "seize, fasten on, hang on (to), cling (to); hang up, suspend," literally "to catch with a hook," from en- "in" (see en- (1)) + croc "hook," from Old Norse krokr "hook" (see crook). Meaning "seize wrongfully" is from c.1400. Sense of "trespass" is first recorded 1530s. Related: Encroached; encroaches; encroaching.
encroachment (n.): 1520s, from encroach + -ment. In Anglo-French from mid-15c.
encrust (v.): 1640s, from French incruster, from Latin incrustare "to cover with crust," from in- (see in- (2)) + crusta (see crust). Related: Encrusted; encrusting.
encrypt (v.): 1975 in computer sense, from en- (1) + crypt (see crypto-). Related: Encrypted; encrypting; encryption.
enculturation (n.): 1948 (Herskovits), from en- (1) + culturation (cf. acculturation).
encumber (v.): early 14c., "burden, vex, inconvenience," from Old French encombrer "to block up, hinder, thwart," from Late Latin incombrare, from in- "in" (see in- (2)) + combrus "barricade, obstacle," probably from Latin cumulus "heap." Meaning "hinder, hamper" is attested in English from late 14c. Related: Encumbered; encumbering.
encumbrance (n.): early 14c., from Old French encombrance, from encombrer (see encumber).
encyclical (adj.): in reference to a letter sent by the Pope to all the bishops, 1640s, from Late Latin encyclicus, from Latin encyclius, from Greek enkyklios "in a circle, circular" (see encyclopedia). As a noun, from 1837.
encyclopaedia (n.): see encyclopedia. The Latin spelling survives as a variant because many of the most prominent ones (e.g. Britannica) have Latin names.
encyclopedia (n.): 1530s, "course of instruction," from Modern Latin encyclopaedia (c.1500), thought to be a false reading by Latin authors of Greek enkyklios paideia taken as "general education," but literally "training in a circle," i.e. the "circle" of arts and sciences, the essentials of a liberal education; from enkyklios "circular," also "general" (from en "in" + kyklos "circle") + paideia "education, child-rearing," from pais (genitive paidos) "child" (see pedo-).

Modern sense of "reference work arranged alphabetically" is from 1640s, often applied specifically to the French "Encyclopédie ou Dictionnaire raisonné des Sciences, des Arts, et des Métiers" (1751-65).