'pro-' originates from Latin, specifically the word 'pro', where 'pro' meant 'for'; 'emperor' originates from Latin, specifically the word 'imperator'.

'emperor' changed from Latin 'imperator' into Old French forms (e.g. 'empereur') and Middle English 'emperour', eventually becoming the modern English word 'emperor'. The prefix 'pro-' remained from Latin into English via direct borrowing.

Initially, 'imperator' in Latin meant 'commander' or 'one who gives orders' and later came to mean a sovereign ruler; the combined sense of 'pro-emperor' has consistently meant 'for the emperor' or 'supporting imperial rule.'