'antireactionary' originates from the prefix 'anti-' (from Greek anti- meaning 'against') combined with 'reactionary' (from French 'réactionnaire'), forming 'antireactionary' to mean 'against reactionaries'.

'reactionary' entered English from French 'réactionnaire' (19th century), from Latin 'reactio' (a doing again). The English adjective 'antireactionary' was formed by adding the prefix 'anti-' to 'reactionary' to indicate opposition; the plural 'antireactionaries' follows standard English pluralization.

Initially formed to denote opposition to 'reactionary' movements or ideas, the term has retained that core meaning and is used to describe people, groups, or positions opposing reactionary politics.