'co-ruler' is a Modern English compound formed from the combining prefix 'co-' and the noun 'ruler'. 'co-' originates from Latin, specifically the word 'com-', where 'com-' meant 'with, together'. 'ruler' originates ultimately from Latin (see below) and is formed in English as the agent noun from the verb 'rule' + the suffix '-er'.

'ruler' entered English via Middle English from Old French forms related to 'reule'/'regle', ultimately from Latin 'regula' (a straight stick, a rule) and the verb 'regere' (to rule). The prefix 'co-' derives from Latin 'com-' and was adopted into English as a productive combining form; together they produced compounds like 'co-king' and later 'co-ruler' in Modern English.

The Latin root 'regula' originally meant 'a straight stick' or 'measuring rod' and then 'a rule' or principle; over time the related words came to mean 'to govern' and 'one who governs'. 'co-ruler' developed to mean specifically 'a ruler who shares power' rather than a sole ruler.