RELENTLESS definition: 1. continuing in a severe or extreme way: 2. continuing in a severe or extreme way: 3. continuing…. Learn more. RELENTLESS meaning: 1 : continuing without becoming weaker , less severe, etc.; 2 : remaining strict or determined The meaning of RELENTLESS is showing or promising no abatement of severity, intensity, strength, or pace : unrelenting . How to use relentless in a sentence. unrelenting : a relentless enemy. re•lent′less•ness, n. rigid, unbending, obdurate, adamant, unyielding. See inflexible. merciful. In Lists: Words ending in "less", New vocabulary, English Words, more... Synonyms: determined, persistent, persevering, dogged, tenacious, more...