'co-magistrate' originates from the English prefix 'co-' (from Latin 'com-') meaning 'with' and from Latin 'magistratus' (via Old French 'magistrat'), where 'magister' meant 'master'.

'magistrate' changed from Latin 'magistratus' to Old French 'magistrat' and then to Middle English 'magistrat(e)', eventually becoming modern English 'magistrate'. The prefix 'co-' entered English from Latin 'com-' and was used productively to form compounds such as 'co-magistrate'.

Initially related to 'master' or senior official in Latin; over time 'magistrate' came to mean a civil or judicial officer, and 'co-magistrate' specifically denotes one who shares that office with another.