'reactively' originates from the word 'reactive', which is derived from the Latin word 'reactivus', where 're-' meant 'again' and 'activus' meant 'active'.

'reactivus' transformed into the English word 'reactive', and eventually became the adverb 'reactively'.

Initially, it meant 'to act in response to something', and this meaning has largely remained the same in modern usage.