A Sisyphean task seems impossible to complete. You might look at the mountain of laundry on your bedroom floor and feel like washing it will be a Sisyphean job — especially since you know that you will have to wash it all again the following week.
You can use Sisyphean to describe things that require a lot of hard work and yet will never be truly finished. Keeping a house clean can feel Sisyphean, and so can deciding on a movie that everyone in a family will enjoy. The word comes from the character Sisyphus in Greek mythology, who was sentenced for his wrongdoing to push a boulder up a hill and watch it roll back down, again and again, forever.