'anti-reactionaries' is formed from the prefix 'anti-' (from Greek 'anti' meaning 'against') + 'reactionary' (from French 'réactionnaire' denoting opposition to progressive change).

'reactionary' comes from French 'réactionnaire', ultimately from Latin 'reactio'/'reactionem' (a 'reaction'); 'anti-' was attached in modern political usage to indicate opposition, producing 'anti-reactionary' and its plural 'anti-reactionaries'.

Initially, 'reactionary' described a person favoring a return to a previous state; combining it with 'anti-' produced a term that simply denotes opposition to such people or ideas. Over time it has been used broadly for political opposition to reactionary movements.