'archreactionary' originates from Modern English, formed by the prefix 'arch-' (from Greek 'archi-' meaning 'chief' or 'extreme') combined with 'reactionary' (from French 'réactionnaire'/'reactionnaire').

'reactionary' entered English from French 'réactionnaire' in the early 19th century, itself from 'réaction' (reaction), from Latin 'reactionem' ('re-' + 'agere' meaning 'to drive' or 'to do'). The intensive prefix 'arch-' (from Greek 'archi-') has been used in English compounds (e.g. 'archconservative', 'archenemy') to mean 'principal' or 'extreme', and was later attached to 'reactionary' to form 'archreactionary'.

Initially 'reactionary' described opposition to political change; the compound 'archreactionary' intensifies that sense to mean 'extremely opposed to change or reform.'