Used when referring to an extremely arduous, endless and/or impossible task.
Sisyphus, the king of Corinth and the son of Aeolus, The Lord of the Winds was very clever, cunning, and deceitful. Zeus kidnapped and raped Aegina, daughter of the river god Asopus. When Asopus was looking for his daughter, Sisyphus disclosed the whereabouts of Aegina to him, greatly angering Zeus in the process for this betrayal.
When the time came for him to be taken to the Underworld, the Lord of the Underworld, Hades went to Corinth personally to claim Sisyphus. Sisyphus had a pair of handcuffs that--which he cunningly expressed so much interest in--Hades was compelled to try on. After putting the handcuffs on, Sisyphus locked Hades in a closet. With the God of Death incapacitated, nobody could die. People would become mortally wounded in battle, but continue to live on. Finally, Hades managed to escape his imprisonment and took Sisyphus to the Underworld--but not before Sisyphus told his wife to throw his unburied body out into the town square. Upon entering the Underworld, he fooled Persephone, the Queen of the Underworld into letting him return to Corinth to chastise his wife for her actions. When he arrived at Corinth, he refused to return to the Underworld.
For his trickery and success in besting the gods, Sisyphus was condemned to Tartarus for all eternity to roll a giant boulder up a hill, only to watch it roll back down. Thus, a Sisyphean Task was born.