Indefatigable

/ˌɪndɪˈfætɪɡəbəl/ adjective hard

Never growing tired, no matter how much effort or time a task requires.

The indefatigable volunteers kept handing out supplies long after everyone else had gone home to rest.

Memory Trick

Split it apart: 'in-' (not) + 'de-fatigue-able' — literally 'not able to be fatigued.'