Etymology
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enemy (n.)
early 13c., "one hateful toward and intent on harming (someone)," from Old French enemi (12c., Modern French ennemi), earlier inimi (9c.) "enemy, adversary, foe; demon, the Devil," from Latin inimicus "an enemy," literally "an unfriend," noun use of adjective meaning "hostile, unfriendly" (source also of Italian nemico, Catalan enamic, Spanish enemigo, Portuguese inimigo), from in- "not" (see in- (1)) + amicus "friend" related to amare "to love" (see Amy). From c. 1300 in English as "adversary of God, unbeliever, heathen, anti-Christian;" late 14c. as "the Devil;" also late 14c. as "member of an armed, hostile body in a war, feud, etc.;" of the opposing military forces as a whole, from c. 1600. From mid-14c. as an adjective.

Most Indo-European words for "personal enemy" cover also "enemy in war," but certain languages have special terms for the latter, such as Greek polemioi (distinct from ekhthroi), Latin hostis, originally "stranger" (distinct from inimicus), Russian neprijatel' (distinct from vrag). Russian vrag (Old Church Slavonic vragu) is cognate with Lithuanian vargas "misery" (see urge (v.)), and probably is related to Proto-Germanic *wargoz, source of Old Norse vargr "outlaw," hence "wolf;" Icelandic vargur "fox;" Old English wearg "criminal, felon;" which likely were the inspirations for J.R.R. Tolkien's warg as the name of a kind of large ferocious wolf in "The Hobbit" (1937) and "Lord of the Rings." Related: Enemies.
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arch-enemy (n.)
also archenemy, 1540s, from arch- + enemy. Originally especially Satan.
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inimical (adj.)

1640s, from Late Latin inimicalis "hostile," from Latin inimicus "unfriendly; an enemy" (see enemy).

Inimical expresses both feeling and action, generally in private affairs. Hostile also expresses both feeling and action, but applies especially to public affairs: where it applies to private matters, it expresses either strong or conspicuous action or feeling, or both, or all. [Century Dictionary, 1902]
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enmity (n.)
late 14c., "hostile feeling, rivalry, malice; internal conflict," from Old French enemite, variant of enemistié "enmity, hostile act, aversion" (Modern French inimité), from Vulgar Latin *inimicitatem (nominative *inimicitas), from Latin inimicitia "enmity, hostility," usually plural, from inimicus "enemy" (see enemy). Related: Enmities. Amity is basically the same word without the negative prefix.
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in- (1)
Origin and meaning of in-
word-forming element meaning "not, opposite of, without" (also im-, il-, ir- by assimilation of -n- with following consonant, a tendency which began in later Latin), from Latin in- "not," cognate with Greek an-, Old English un-, all from PIE root *ne- "not."

In Old French and Middle English often en-, but most of these forms have not survived in Modern English, and the few that do (enemy, for instance) no longer are felt as negative. The rule of thumb in English has been to use in- with obviously Latin elements, un- with native or nativized ones.
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fiend (n.)

Old English feond "enemy, foe, adversary," originally present participle of feogan "to hate," from Proto-Germanic *fijand- "hating, hostile" (source also of Old Frisian fiand "enemy," Old Saxon fiond, Middle Dutch viant, Dutch vijand "enemy," Old Norse fjandi, Old High German fiant, Gothic fijands), from suffixed form of PIE root *pe(i)- "to hurt" (source also of Sanskrit pijati "reviles, scorns;" Avestan paman-, name of a skin disease; Greek pema "disaster, sorrow, misery, woe;" Gothic faian "to blame").

As spelling suggests, the word originally was the opposite of friend (n.). Both are from the active participles of the Germanic verbs for "to love" and "to hate." Boutkan says the "fiend" word was a Germanic analogical formation from the "friend" word. According to Bammesberger ["English Etymology"], "The long vowel in FIEND is regular. In FRIEND the vowel has been shortened; perhaps the shortening is due to compounds like FRIENDSHIP, in which the consonant group (-nds-) regularly caused shortening of the preceeding long vowel."

Fiend at first described any hostile enemy (male and female, with abstract noun form feondscipe "fiendship"), but it began to be used in late Old English for "the Devil, Satan" (literally "adversary") as the "enemy of mankind," which shifted its sense to "diabolical person" (early 13c.). The old sense of the word devolved to foe, then to the imported word enemy. For spelling with -ie- see field. Meaning "devotee (of whatever is indicated)," as in dope fiend, is from 1865.

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hostile (adj.)

late 15c., from French hostile "of or belonging to an enemy" (15c.) or directly from Latin hostilis "of an enemy, belonging to or characteristic of the enemy; inimical," from hostis, in earlier use "a stranger, foreigner," in classical use "an enemy," from PIE root *ghos-ti- "stranger, guest, host." The noun meaning "hostile person" is recorded from 1838, American English, a word from the Indian wars. Related: Hostilely.

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adversary (n.)
"unfriendly opponent, enemy" (originally especially of Satan as the enemy of mankind), mid-14c., aduersere, from Anglo-French adverser (13c.), Old French adversarie (12c., Modern French adversaire) "hostile opponent, enemy," or directly from Latin adversarius "an opponent, rival, enemy," noun use of adjective meaning "opposite, hostile, contrary," literally "turned toward one," from adversus "turned against, turned toward, fronting, facing," figuratively "hostile, adverse, unfavorable," past participle of advertere "to turn toward," from ad "to" (see ad-) + vertere "to turn, turn back, be turned; convert, transform, translate; be changed" (see versus). The Latin word is glossed in Old English by wiðerbroca.
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hater (n.)
"one who hates, an enemy," late 14c., agent noun from hate (v.).
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plunder (n.)

"goods taken from an enemy by force; act or action of plundering," 1640s, from plunder (v.).

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