Indefatigable (adjective)

1. Incapable of being fatigued; untiring.
2. Persistent and untiring in working towards a goal; determined.

Origin:

From french, from in- + fatigable ("that can be fatigue.d")

Examples:

1. She was an indefatigable worker, always the last to leave the office.
2. He was an indefatigable campaigner, working tirelessly to get the vote out.
3. The indefatigable athlete won the marathon despite the heat.
4. The indefatigable researcher spent hours in the lab.
5. The indefatigable teacher was always willing to go the extra mile for her students.