sword of Damocles

From the following story:

Damocles was an obsequious courtier in the court of Dionysius II of Syracuse, a fourth century BC tyrant of Syracuse. Damocles exclaimed that, as a great man of power and authority, Dionysius was truly fortunate. Dionysius offered to switch places with him for a day, so he could taste that fortune first-hand. In the evening a banquet was held, where Damocles very much enjoyed being waited upon like a king. Only at the end of the meal did he look up and notice a sharpened sword hanging directly above his head, held only by a single horse-hair. Immediately, he lost all taste for the festivities and asked leave of the tyrant, saying he no longer wanted to be so fortunate. Dionysius had successfully conveyed a sense of the constant threat under which a powerful man lives.

From Ancient Greek Δαμοκλῆς (Damoklês).

• (General American) IPA^(key): /ˈsɔɹd əv ˈdæməkliz/

sword of Damocles (plural swords of Damocles)

1. A thing or situation which causes a prolonged state of impending doom or misfortune.
   • 1961 December 10, quoting John F. Kennedy, “‘Sword of Damocles’ Popular These Days”, in The New York Times‎, →ISSN, page 22:

     Every man, woman and child lives under a nuclear sword of Damocles hanging by the slenderest of threads, capable of being cut at any moment by accident, or miscalculation or by madness. The weapons of war must be abolished before they abolish us.

• hang by a hair
• hang by a thread