(A). A present which seems valuable, but which is in reality a curse; as when Midas was permitted, according to his request, to turn whatever he touched into gold, and found his very food became gold, and therefore uneatable. Prometheus made an image and stole fire from heaven to endow it with life. In revenge, Jupiter told Vulcan to make a female statue, and gave her a box which she was to present to the man who married her. Prometheus distrusted Jove and his gifts, but Epimetheus, his brother, married the beautiful Pandora, and received the box. Immediately the bridegroom opened the box all the evils that flesh is heir to flew forth, and have ever since continued to afflict the world. The last thing that flew from the box was Hope.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
More on Pandora's Box from Fact Monster: