"tireless" in the adjective sense
1. indefatigable, tireless, unflagging, unwearying
showing sustained enthusiastic action with unflagging vitality
"an indefatigable advocate of equal rights"
"a tireless worker"
"unflagging pursuit of excellence"
2. hardworking, industrious, tireless, untiring
characterized by hard work and perseverance
Source: WordNet® (An amazing lexical database of English)
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