'pro-unionism' is formed from the prefix 'pro-' (from Latin 'pro' meaning 'for') + 'union' (from Latin 'unio', from 'unus' meaning 'one') + the suffix '-ism' (from Greek '-ismos' via Latin '-ismus'), together denoting the doctrine or stance of being for a union.

'union' passed into English via Old French 'union' from Latin 'unio.' The suffix '-ism' entered English via Latin and French. The compound 'pro-union' and the abstract noun 'pro-unionism' emerged in modern English to express a position 'for union' (attested in the 19th–20th centuries in labor and political contexts).

Originally 'union' simply meant 'the state of being one or joined.' Over time 'unionism' developed specialized senses — especially (1) organized labor advocacy and (2) political support for maintaining a national/political union — and 'pro-unionism' came to mean actively supporting those specific senses.