What is the X in X-mas?
King of Corinth, famed as "the craftiest of men," he was condemned in the afterlife to roll uphill a stone which perpetually rolls down again; Greek Sisyphos, a name of unknown origin. Liddell and Scott suggest a reduplication of syphos "the crafty" (with Aeolic -u- for -o-), but Klein calls this folk-etymology.
A king in classical mythology who offended Zeus and was punished in Hades by being forced to roll an enormous boulder to the top of a steep hill. Every time the boulder neared the top, it would roll back down, and Sisyphus would have to start over.
Note: A difficult and futile endeavor may be called a “labor of Sisyphus” or a “Sisyphean task.”