The Collaborative International Dictionary of English v.0.48:

Sisyphus \Sis"y*phus\, n. [L. Sisyphus, Sisyphus, fr. Gr. ????.]
   (Class. Myth.)
   A king of Corinth, son of Aeolus, famed for his cunning. He
   was killed by Theseus, and in the lower world was condemned
   by Pluto to roll to the top of a hill a huge stone, which
   constantly rolled back again, making his task incessant.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Sisyphus
    n 1: (Greek legend) a king in ancient Greece who offended Zeus
         and whose punishment was to roll a huge boulder to the top
         of a steep hill; each time the boulder neared the top it
         rolled back down and Sisyphus was forced to start again