'reactionarily' originates from French, specifically the word 'réactionnaire' (reactionary), ultimately from Latin 'reactio' where the prefix 're-' meant 'back' and the root from 'agere' meant 'to do or drive.'

'reactio' (Latin) → Old French/Medieval Latin forms (réaction) → French 'réaction' and adjectival 'réactionnaire' → English 'reactionary' → adverbial formation 'reactionarily'.

Initially related to the idea of a 'doing back' or a return (reaction); it later came to denote political or social opposition to change, and this core sense persists in the modern adverb 'reactionarily' meaning 'in a manner opposing progress.'