You can think of unrelenting as a hand that will not open. To relent is to your hold; un- takes that away, so nothing is ever let go of, not even for a second. The word is not really about how strong the grip is — it is about the fact that no gap ever appears in it, which is why the things it describes are so .
unrelenting 可以想成一隻抓著不放的手。relent 是鬆手、放軟，前面加 un- 就成了「從來不鬆手」。它說的重點其實不是力氣多大，而是那隻手一刻也沒有放開，所以被抓住的人連喘一口氣的縫都找不到。
From the word’s history
Wiktionary gives the word as un- + relenting, so the negative is doing all the work: not loosening, not giving way.
來源資料把這個字拆成 un- 加 relenting，意思全靠那個否定：不放軟、不讓步，也就是不鬆手。
Wiktionary, "unrelenting"
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