'anti-unionism' is built from the prefix 'anti-' (from Greek 'anti' meaning 'against'), the noun 'union' (from Latin 'unio' via Old French 'union', meaning 'oneness' or 'unity'), and the suffix '-ism' (from Greek/Latin '-ismos/-ismus' denoting a practice, system, or ideology).

'union' passed from Latin 'unio' (from 'unus' meaning 'one') into Old French as 'union' and then into Middle English; the prefix 'anti-' was borrowed from Greek into English as a productive prefix; combining these elements produced phrases like 'anti-union' and the later abstract noun 'anti-unionism'.

Originally elements meant 'against' + 'oneness' (in a literal sense), but in modern usage 'anti-unionism' has come to mean specifically opposition to labor unions and their organization.