'unionize' originates from Modern English formation combining the noun 'union' and the verb-forming suffix '-ize' (the suffix ultimately from Greek '-izein' through Latin and French); 'union' comes from Latin 'unio' meaning 'unity, oneness'.

'union' passed into English via Old French 'union' from Latin 'unio'; in modern English the suffix '-ize' was attached to form 'unionize' (to make into a union or organize into a union).

Initially, the components meant 'unity' (from 'unio') and the combined form meant 'to make into a union'; over time it maintained this sense and came to specifically mean organizing workers into a labor union.