Unflagging describes effort, energy, interest, or support that keeps going at full strength and never gets tired or weaker. The key is the word 'never' — it is not about being strong once, it is about being strong for a long time, hour after hour, year after year. So it almost always goes with words about long stretches of effort: unflagging support, unflagging energy, unflagging enthusiasm, unflagging determination, unflagging devotion. Think of a nurse who works a twelve-hour night and is still gentle with the last patient, or a mother who has helped her son with homework every evening for six years. It is a warm word. When you call someone's effort unflagging, you are praising them, and you sound a little formal and a little moved. You meet it in speeches, in thank-you letters, in newspaper writing, in the first page of a book where the writer thanks their family. In everyday talk people usually just say 'she never gives up' or 'he's always full of energy'. One thing to notice: it describes the effort, not usually the person. We say 'his unflagging support', and 'an unflagging worker' is possible but much rarer. Also, it is one of those un- words where the positive form is nearly dead — English keeps 'unflagging' but almost never uses 'flagging' as a compliment, because 'flagging' by itself means getting weak (flagging sales, flagging interest, my energy was flagging by three o'clock). So the pair is a bit lopsided: 'flagging' is common enough, 'unflagging' is common enough, but they live in different kinds of sentences.

不懈的、不減的（形容熱情、支持、努力一直維持在最強，從不變弱）；常見說法：不遺餘力的支持、始終不渝的熱情。注意這裡的 flag 跟「旗子」沒有關係，是動詞「變弱、鬆懈」的意思。