en·e·my / ˈenəmē/ • n. (pl. -mies) a person who is actively opposed or hostile to someone or something. ∎  (the enemy) [treated as sing. or pl.] a hostile nation or its armed forces or citizens, esp. in time of war: the enemy shot down four helicopters [as adj.] enemy aircraft. ∎  a thing that harms or weakens something else: routine is the enemy of art.ORIGIN: Middle English: from Old French enemi, from Latin inimicus, from in- ‘not’ + amicus ‘friend.’

enemy be one's own worst enemy act in a way contrary to one's own interests. It is said that the Labour politician Ernest Bevin (1881–1951), on hearing the comment that Aneurin Bevan was sometimes his own worst enemy, responded, ‘Not while I'm alive 'e ain't!’ (The comment is also attributed to Bevin of Herbert Morrison.)
the enemy of my enemy is my friend modern proverbial saying, sometimes said to be of Arab origin, and frequently used in discussions of western diplomacy in the Middle East.

See also the best is the enemy of the good, it is good to make a bridge of gold to a flying enemy, the good is the enemy of the best, there is no little enemy, public enemy number one.

217. Enemy

1. Amalekites Israel’s hereditary foe and symbol of perpetual hatred. [Jew. Hist.: Wigoder, 24]
2. Antichrist principal antagonist of Christ. [Christianity: NCE, 117]
3. Armilus legendary name of anti-Messiah. [Judaism: Wigoder, 41]
4. Satan also called the Adversary or the Devil. [Christianity: Misc.]

enemy XIII. — OF. enemi (mod. ennemi) :- L. inimīcus, f. IN-2 + amīcus friend.