Indefatigable

Part of speech: adjective (a.)

Incapable of being fatigued; not readily exhausted; unremitting in labor or effort; untiring; unwearying; not yielding to fatigue; as, indefatigable exertions, perseverance, application. "A constant, indefatigable attendance." South. Upborne with indefatigable wings. Milton.

Synonyms: Unwearied; untiring; persevering; persistent.