'merciless' originates from Middle English, specifically the word 'merciles,' where 'merci' meant 'pity' and '-less' meant 'without.'

'merciles' transformed into the modern English word 'merciless' by the addition of the suffix '-ness' to form 'mercilessness.'

Initially, it meant 'without pity,' and this meaning has largely remained the same in modern usage.