'thankless' originates from Old English, specifically the word 'þancleas', where 'þanc' meant 'thought, gratitude' and '-leas' meant 'without'.

'þancleas' transformed into the Middle English word 'thankles', and eventually became the modern English word 'thankless'.

Initially, it meant 'without gratitude', but over time it evolved into its current meaning of 'not likely to be appreciated or rewarded'.