Origins
The English word 'enemy' is a French loanword that arrived after the Norman Conquest, displacing a perfectly serviceable native term and in the process reshaping the semantic landscape of hostility in English. It comes from Old French 'enemi' (Modern French 'ennemi'), which descends from Latin 'inimīcus,' a straightforward compound of 'in-' (not) and 'amīcus' (friend, from 'amāre,' to love). At its etymological core, an enemy is simply a 'not-friend' — a person with whom the bond of love or friendship does not exist or has been broken.
The Latin distinction between 'inimīcus' and 'hostis' is worth understanding, because it left traces in English. 'Inimīcus' referred to a personal enemy — someone who bore you ill will as an individual. 'Hostis' originally meant 'stranger' or 'foreigner' and came to mean a public enemy, an enemy of the state (the source of English 'hostile,' 'host' in the military sense, and 'hostage'). When French 'enemi' entered English, it carried primarily the 'inimīcus' sense of personal enmity, though it could also be used for national enemies, especially in the phrase 'the enemy' referring to an opposing army.
The word 'enemy' displaced Old English 'fēond' from its primary meaning of 'enemy, adversary.' 'Fēond' was the present participle of the verb 'fēon' (to hate), literally meaning 'a hating one' — the structural mirror of 'frēond' (friend, literally 'a loving one'). After the French word took over the general sense, 'fēond' narrowed dramatically, becoming Modern English 'fiend' — now used almost exclusively for the Devil or for a person of extreme wickedness or obsession ('a dope fiend'). This semantic shift from 'enemy' to 'demon' was facilitated by the medieval Christian use of 'the Fiend' as a name for Satan, who was understood as the ultimate enemy of humanity.
Proto-Indo-European Roots
The Latin source word 'amīcus' (friend) itself has a disputed deeper etymology. The standard derivation connects it to 'amāre' (to love), but the origin of 'amāre' is uncertain. Some scholars have proposed a PIE root *am- (a nursery word for mother or endearment), while others suggest a Mediterranean substrate origin. What is clear is that Latin built a systematic vocabulary of friendship and enmity around this root: 'amīcus' (friend), 'amīcitia' (friendship), 'amābilis' (lovable, source of English 'amiable'), 'inimīcus' (enemy), 'inimīcitia' (enmity).
English absorbed derivatives from both sides of this Latin pair. From the positive 'amīcus' came 'amicable' (friendly, agreeable), 'amity' (friendship between nations), and 'amiable' (pleasant, likeable). From the negative 'inimīcus' came 'enemy' itself, 'enmity' (the state of being enemies, from Old French 'enemistié'), and 'inimical' (hostile, unfavorable — a learned borrowing directly from Latin that entered English in the sixteenth century, long after 'enemy' had been naturalized).
The phonological journey from Latin 'inimīcus' to English 'enemy' follows the regular patterns of French sound change and English adaptation. The Latin initial 'in-' remained, but the unstressed interior syllables were compressed: 'inimīcus' > Late Latin 'inimīcus' > Old French 'enemi' (with the typical French opening of Latin 'i' to 'e' in initial syllables, and the loss of the final Latin inflection). English borrowed the French form directly, with minimal further phonological change.
Cultural Impact
English possesses an unusually rich vocabulary for the concept of 'enemy,' drawing from multiple etymological layers. Beyond 'enemy' (French/Latin), there is 'foe' — from Old English 'fāh' (hostile, at feud, outlawed), from Proto-Germanic *faihaz, possibly from PIE *peyḱ- (hostile). 'Foe' is more native, more poetic, and carries a slightly archaic flavor. There is 'adversary' — from Latin 'adversārius' (one turned against), from 'advertere.' There is 'antagonist' — from Greek 'antagōnistḗs' (opponent, rival). And there is the remnant of the oldest native term, 'fiend,' now restricted to supernatural evil. This layering of synonyms from different source languages is characteristic of English's absorptive vocabulary-building, where new borrowings do not fully replace old words but push them into specialized niches.
The theological weight of 'enemy' in English literature is considerable. The Vulgate Bible's 'diligite inimīcos vestrōs' (love your enemies, Matthew 5:44) was translated in Middle English with the newly borrowed 'enemy,' and this passage has given the word a particular resonance in Christian moral discourse. The instruction to love one's 'inimīcus' — one's not-friend — is etymologically a command to extend the bond of love precisely to those who are defined by its absence. The paradox built into the Latin compound becomes a theological imperative.