Sisyphean • \sis-uh-FEE-un\  • adjective
: of, relating to, or suggestive of the labors of Sisyphus; specifically : requiring continual and often ineffective effort

Example sentence:
"Considering the inherently fickle nature of golf, defending a title is, no doubt, as Sisyphean a task as any in the sport." (Anthony Cuaycong, BusinessWorld, July 2008)

Did you know?
In Greek mythology, Sisyphus was a king who annoyed the gods with his trickery. As a consequence, he was condemned for eternity to roll a huge rock up a long, steep hill in the underworld, only to watch it roll back down. The story of Sisyphus is often told in conjunction with that of Tantalus, who was condemned to stand beneath fruit-laden boughs, up to his chin in water. Whenever he bent his head to drink, the water receded, and whenever he reached for the fruit, the branches moved beyond his grasp. Thus to "tantalize" is to tease or torment by offering something desirable but keeping it out of reach -- and something "Sisyphean" (or "Sisyphian," pronounced \sih-SIFF-ee-un\) demands unending, thankless, and ultimately unsuccessful efforts.