Evil foreboded or dreaded. Damocles, the sycophant of Dionysius the elder, of Syracuse, was invited by the tyrant to try the felicity he so much envied. Accordingly he was set down to a sumptuous banquet, but overhead was a sword suspended by a hair. Damocles was afraid to stir, and the banquet was a tantalising torment to him. (Cicero.)
“These fears hang like Damocles' sword over every feast,and make enjoyment impossible.” —Chambers's Cyclopædia.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894