'self-reactiveness' originates from Modern English, specifically the compound of the prefix 'self-' and the adjective 'reactive', where 'self-' (Old English 'self') meant 'oneself' and 'reactive' derives from 'react' (from Latin roots) meaning 'to act back' or 'respond'.

'react' comes from Latin roots 're-' ('again, back') + 'agere' ('to do, drive'); it passed into English via Late Latin/France-derived forms and gave rise to the adjective 'reactive' in the 19th century. The prefix 'self-' is Old English 'self'; the compound 'self-reactive' is a modern English formation, and adding the noun-forming suffix '-ness' produced 'self-reactiveness'.

Initially the components signified 'oneself' + 'able to react' (i.e. 'able to react on its own'); over time the noun form 'self-reactiveness' has come to denote the property or degree of that ability (from literal chemical/material senses to occasional figurative uses).