Synonyms for Unflagging (same or very similar meaning)
WordNet sense 1 (showing sustained enthusiastic action with unflagging vitality):
indefatigable, tireless, unwearying
WordNet sense 2 (possessing or exerting or displaying energy):
energetic
WordNet sense 3 (unceasing):
unfailing
WordNet sense 4 (steadfast in purpose or devotion or affection):
constant
From the ODE community, based on WordNet
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