'reactively-formed' originates from the combination of 'reactive' and 'formed', where 'reactive' is derived from 'react', meaning 'to respond to a stimulus', and 'formed' from 'form', meaning 'to shape or create'.

'Reactive' evolved from the Latin word 'reactivus', and 'formed' from the Old English 'formian'.

Initially, 'reactively-formed' meant 'created in response to a chemical reaction', but it has expanded to include any formation resulting from a reaction.