'thoughtlessly' originates from the word 'thoughtless,' which is derived from the Old English word 'þohtleas,' where 'þoht' meant 'thought' and '-leas' meant 'without.'

'þohtleas' transformed into the Middle English word 'thoughtles,' and eventually became the modern English word 'thoughtless.'

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