'pro-medieval' is a compound formed from the prefix 'pro-' (from Latin 'pro', meaning 'for' or 'in favor of') and the adjective 'medieval' (pertaining to the Middle Ages).

'medieval' comes from Medieval Latin 'medium aevum' meaning 'middle age'; it entered English via Middle French and Middle English as forms meaning 'of the Middle Ages', and later combined with modern prefixes (like 'pro-') to form compounds such as 'pro-medieval'.

Originally 'medieval' simply denoted things 'relating to the Middle Ages'; the compound 'pro-medieval' specifically adds the sense 'in favor of' by using the prefix 'pro-', so its meaning is now 'in favor of medieval (things)'.