The labor of Sisyphus refers to a task that's never-ending and repetitive, similar to the Greek mythological tale of Sisyphus who was forced to push a boulder up a hill only to watch it roll back down again. Synonyms for this phrase include a sisyphean task, an endless drudgery, a never-ending cycle, a futile chore, an unremitting burden, and a perpetual grind. Other alternatives include an unending toil, a monotonous undertaking, a ceaseless struggle, a tireless effort, a continuous labor, and a wearisome routine. The labor of Sisyphus represents an arduous task, one that is pointless and repetitive, making it a challenging and frustrating endeavor.