"unflagging" 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. unfailing, unflagging
unceasing
"unfailing loyalty"
"unfailing good spirits"
"unflagging courtesy"
Source: WordNet® (An amazing lexical database of English)
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