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English word relentless comes from English relent (a dated word meaning to stay, stop or delay) and -less (a suffix which denotes a lack of some quality)
relent (English)
Stay; stop; delay. (dated) To become less rigid or hard; to soften; to yield; to dissolve; to melt; to deliquesce.. (obsolete, transitive) To lessen, make less severe or fast.. To become less severe or intense; to become less hard, harsh, or cruel; to soften in temper; to become more mild and tender; to feel compassion.. To slacken; to abate.
-less (English)
Lacking (something); without (something).
relentless (English)
Unrelenting or unyielding in severity. Unremitting, steady and persistent.