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

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

Initially, it meant 'to become less severe,' but over time it evolved into its current meaning of 'unyielding or persistent.'