'pro-royal' is a compound formed from the prefix 'pro-' and the adjective 'royal'. 'pro-' originates from Latin, specifically the word 'pro', where 'pro' meant 'for'. 'royal' originates from Old French, specifically the word 'roial', which in turn comes from Latin 'regalis'.

'royal' changed from Old French 'roial' into Middle English 'royal' and eventually became the modern English word 'royal'. The prefix 'pro-' was borrowed from Latin into English as a productive prefix meaning 'for' or 'in favor of', and in modern English these elements combined to form the compound 'pro-royal'.

Initially, 'royal' meant 'kingly' (from Latin 'regalis') and 'pro-' meant 'for'; over time the compound 'pro-royal' evolved to mean 'in favor of the monarchy' or 'supportive of the royal family'.