'unflaggingly' originates from the English word 'unflagging', where 'un-' is a prefix meaning 'not', and 'flagging' comes from 'flag', meaning 'to become weak or tired'.

'unflagging' transformed from the Middle English word 'flaggen', which meant 'to droop or become weak', and eventually became the modern English word 'unflagging'.

Initially, it meant 'not becoming weak or tired', and this meaning has largely remained the same in modern usage.