'antiunionist' originates from English, specifically the prefix 'anti-' and the noun 'unionist', where 'anti-' meant 'against' and 'unionist' meant 'one who supports or belongs to a union.'

'union' comes from Latin 'unio' via Old French 'union' and Middle English 'unioun'; 'unionist' was formed by adding the agent suffix '-ist' to 'union' (19th century usage); 'antiunionist' was created by prefacing 'unionist' with 'anti-' to indicate opposition, becoming common in the late 19th to 20th century in labor contexts.

Initially the components meant 'against' + 'one who supports a union,' and together they have consistently meant 'someone opposed to unions' or 'opposed to union activity' in modern usage.