Someone who is indefatigable can go on for a very long time without becoming tired. You might not be so happy to have an indefatigable guide on your walking trip––you'll have blisters, but she'll see no reason not to keep going.

Indefatigable comes from Latin indefatigabilis, formed from the prefix in- "not" plus defatigare "to tire out." Here the prefix de- means "entirely." You can remember the root fatigare because it sounds so much like the English fatigue.

Definitions of indefatigable
  1. adjective
    showing sustained enthusiastic action with unflagging vitality
    “an indefatigable advocate of equal rights”
    synonyms: tireless, unflagging, unwearying
    possessing or exerting or displaying energy
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