not easily tired or beaten
indefatigable - tireless
To be ‘indefatigable’ is to be tireless in your efforts and determined to never give up. This formal word implies a high level of mental strength and inner energy that can push people through even the most difficult of circumstances. Whether in a social or professional context, this term has only positive connotations.
Emily is quite indefatigable. No matter how difficult things become, she just keeps on going relentlessly.
The illness took a lot out of John, but the indefatigableness of his spirit meant that he persevered and made a full recovery.
Kevin was awarded a bonus because of his indefatigable, and ultimately successful, efforts to secure new funding for the project over the past few months.