'rightfully' originates from the Old English word 'rihtful,' where 'riht' meant 'just' or 'lawful' and '-ful' was a suffix indicating 'full of.'

'rihtful' transformed into the Middle English word 'rightful,' and eventually became the modern English word 'rightfully.'

Initially, it meant 'in a manner full of justice or lawfulness,' and this meaning has largely remained the same in modern usage.