Source : Webster's Revised Unabridged Dictionary (1913)

Sisyphean \Sis`y*phe"an\, a.
   Relating to Sisyphus; incessantly recurring; as, Sisyphean
   labors.

Source : WordNet®

Sisyphean
     adj 1: of or relating to Sisyphus
     2: both extremely effortful and futile