Here are some synonyms for relentless:
* Unrelenting
* Persistent
* Inflexible
* Unyielding
* Implacable
* Ceaseless
* Unending
* Tireless
* Indefatigable
Examples:
* The relentless heat of the desert was unbearable.
* He was relentless in his pursuit of justice.
* Her relentless optimism inspired her teammates.
Origin:
The word "relentless" comes from the Old English word "relentan," meaning "to soften, relax, slacken." It is often used to describe something that is persistent and unstoppable, like a force of nature or a determined individual.