'relentless' originates from the Latin word 'lentus,' meaning 'slow, flexible,' combined with the prefix 're-' indicating 'again' or 'back.'

'relentless' evolved from the Middle English word 'relenten,' which meant 'to soften or yield,' and eventually became the modern English word 'relentless.'

Initially, it meant 'to soften or yield,' but over time it evolved into its current meaning of 'unyielding or persistent.'