indefatigable \in-dih-FAT-ih-guh-bul\, adjective:

Incapable of being fatigued; not readily exhausted; untiring; unwearying; not yielding to fatigue.

She was always seeking to add to her collection and was an indefatigable first-nighter at Broadway shows.
-- Meryle Secrest, Stephen Sondheim: A Life