Never getting tired or giving up, even when doing something for a very long time.
From Old English 'tyr' meaning 'fatigue' combined with the suffix '-less' meaning 'without'. The word evolved to describe someone with endless energy and determination.
Victorian factory workers used this word to describe machines that could run 24/7, but we borrowed it to praise humans who show superhuman persistence—now it's one of the highest compliments you can give an athlete or activist.
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