'anti-reactionary' is a compound formed from the prefix 'anti-' (from Greek 'antí-' meaning 'against, opposed to') and the adjective 'reactionary' (from 'reaction' + suffix '-ary').

'reactionary' entered English in the early 19th century from French 'réactionnaire' (from 'réaction'), while the prefix 'anti-' comes from Greek via Latin and Old French; the compound 'anti-reactionary' arose in English in political contexts to label opposition to reactionary forces, particularly in the 19th and 20th centuries.

Initially, 'reactionary' described forces favoring a return to a previous social or political state; 'anti-reactionary' developed to mean 'against such forces.' The core sense—opposition to reactionary politics—has remained stable, though usage has broadened to general opposition to regressive tendencies.