'ultra-reactionary' is a compound formed from the prefix 'ultra' and the adjective 'reactionary'. 'Ultra' originates from Latin, specifically the word 'ultra', where 'ultra' meant 'beyond'. 'Reactionary' originates from French 'réactionnaire', from 'réaction' (from Latin 'reactio'), where 're-' meant 'again' or 'back' and 'actio' meant 'action'.

'Reactionary' entered English via French 'réactionnaire' in the late 18th to early 19th century to describe opposition to revolutionary or progressive change; the prefix 'ultra-' has been used in English since the 19th century to intensify adjectives. Combined, 'ultra-reactionary' developed to denote a position beyond ordinary reactionary views—i.e., extremely opposed to reform.

Initially, 'reactionary' referred to opposition to revolutionary change and to restoring earlier conditions; over time the compounded form 'ultra-reactionary' came to mean 'extremely reactionary' or 'far beyond ordinary conservatism' in modern usage.