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).
Relentless detailed word origin explanation
|relent || English (eng) ||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 (eng) ||Lacking (something); without (something). |
|relentless || English (eng) ||Unrelenting or unyielding in severity. Unremitting, steady and persistent. |
Words with the same origin as relentless