'disunite' originates from Latin elements: the prefix 'dis-' (from Latin 'dis-') meaning 'apart, away' and 'unite' (from Latin 'unitus' / 'unire') where 'unus' meant 'one'.

'dis-' + 'unite' was formed in English by prefixing Latin 'dis-' to the verb 'unite' (which came into English via Old French 'unir' from Latin 'unire'); this produced 'disunite' in post-medieval English as a straightforward negative or reversing form of 'unite'.

Initially it expressed the literal idea of reversing or undoing union ('to make not one'); over time it has retained that core sense and has also been used more broadly for causing division, disagreement, or lack of cooperation.