Quiz: Remember the definition of mal de mer?
early 13c., from Old French enemi (12c.), earlier inimi (9c.) "enemy, adversary, foe, demon, the Devil," from Latin inimicus "hostile, unfriendly; an enemy" (source of Italian nemico, Catalan enamic, Spanish enemigo, Portuguese inimigo), from in- "not" (see in- (1)) + amicus "friend" related to amare "to love" (see Amy).
Most words for "personal enemy" cover also "enemy in war," but certain languages have special terms for the latter, e.g. 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), 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 "a kind of large ferocious wolf" in "The Hobbit" (1937) and "Lord of the Rings." Related: Enemies.