'reactively-built' originates from the combination of 'reactive' and 'built'. 'Reactive' comes from the Latin word 'reactivus', meaning 'to act in response', and 'built' is the past participle of 'build', from Old English 'byldan', meaning 'to construct'.

'Reactive' evolved from the Latin 'reactivus' through Middle French 'réactif', and 'built' from Old English 'byldan' to the modern English 'build'.

Initially, 'reactive' meant 'to act in response', and 'built' meant 'constructed'. Together, they imply a construction made in response to a situation.