In a manner showing no signs of fatigue; with persistent energy and determination without becoming weary.
From 'tire' (to become weary) + suffix '-less' (without) + adverbial '-ly'. The verb 'tire' comes from Old English 'teorian' meaning to fail or cease. The concept of being 'without tiring' emerged in the 16th century.
This word captures the paradox of human determination - while our bodies are designed to tire as a survival mechanism, our greatest achievements often come from pushing beyond those natural limits through sheer will.
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