The definition of sisyphean is a task that is endless and cannot ever be completed.(adjective)
An example of a Sisyphean task is a task that will never be done no matter how long you work at it.
See Sisyphean in Webster's New World College Dictionary
Origin: < L Sisypheius < Gr Sisypheios < Sisyphos + -an
See Sisyphean in American Heritage Dictionary 4
Origin: From Latin Sisyphēius
Origin: , from Greek Sisupheios
Origin: , from Sisuphos, Sisyphus.