An imminent and/or constant threat. According to Greek legend, Damocles, a friend of King Dionysius of Syracuse, envied the ruler's great wealth and power. When Damocles told the king how fortunate he was, Dionysius offered to change places for a day. As Damocles dined at the head of the table, he happened to look up. There above his head, held by only a single horsehair, hung a sharp sword pointing downward toward his chair. Frozen with fear that the thread would break, he pointed out the predicament to the king. Dionysius nodded, acknowledging that the sword was a constant factor in his life, an actual and a metaphoric reminder that some person or circumstance might at any time cut the thread. Such risk, the king added, comes as an integral part of power. Any ever-present risk, especially one that's hanging by a thread, is how the phrase has been used.