'reactivator' originates from Modern English, specifically formed from the verb 'reactivate' plus the agent suffix '-or', where 're-' meant 'again' and 'activate' meant 'to make active'.

'reactivate' itself was formed in English by prefixing Latin-derived 're-' to 'activate' (from Latin 'activare', from 'activus' meaning 'active'); the agent noun 'reactivator' was later formed by adding '-or' (an agent-forming suffix of Latin origin).

Initially the components meant 'again' (re-) and 'make active' (activate); over time the compound came to mean specifically 'an agent that makes something active again' and, in technical contexts, a substance that restores biochemical activity.