Dictionary:

Sisyphean

  (sĭs'ə-fē'ən)

1. Greek Mythology. Of or relating to Sisyphus.
2. Endlessly laborious or futile: “Their patients' lack of education and the high cost of medicine make health care a Sisyphean task” (Frank Gibney, Jr.).

[From Latin Sisyphēius, from Greek Sisupheios, from Sisuphos, Sisyphus.]

The adjective has 2 meanings:

Meaning #1: of or relating to Sisyphus
  Pertains to noun: Sisyphus (meaning #1)

Meaning #2: both extremely effortful and futile