From relent + -less.

• IPA: /ɹɪˈlɛntləs/

relentless

1. Unrelenting or unyielding in severity.
   • 1930, Norman Lindsay, Redheap, Sydney, N.S.W.: Ure Smith, published 1965, →OCLC ↗, page 111:

     She sniffed, too, comprehendingly, and fixed her son with a relentless eye.

2. Unremitting, steady and persistent.

• French: implacable, impitoyable
• German: unnachgiebig, unbarmherzig, unnachsichtig, unerbittlich
• Italian: implacabile
• Portuguese: implacável
• Russian: неумоли́мый
• Spanish: implacable, furioso

• French: tenace
• German: unablässig, unermüdlich
• Italian: tenace
• Portuguese: inexorável
• Russian: упо́рный
• Spanish: inexorable, incansable