'thanklessness' originates from the Old English word 'þancian,' where 'þanc' meant 'thought' or 'gratitude.'

'þancian' transformed into the Middle English word 'thank,' and eventually became the modern English word 'thanklessness' by adding the suffix '-less' and '-ness.'

Initially, it meant 'lack of gratitude,' and this meaning has largely remained the same in modern usage.