'nonreactivity' originates from Modern English, formed by the negative prefix 'non-' (from Latin/Old French 'non', meaning 'not') and the noun 'reactivity', which ultimately derives from Latin elements 're-' ('again') and 'agere' ('to do' or 'to drive').

'react' developed from Latin 're-' + 'agere' into Vulgar/Old French forms (e.g. 'réagir') and then into English as 'react'; 'reactivity' was formed in English by adding the suffix '-ivity' to 'reactive', and 'nonreactivity' is the Modern English formation by prefixing 'non-' to 'reactivity'.

Originally the root idea meant 'to act back' or 'to do again'; over time 'reactivity' came to mean the tendency to respond (physically or emotionally), and 'nonreactivity' developed to denote the absence of such responsiveness in psychological or chemical contexts.