'pro-' originates from Latin, specifically the word 'pro', where 'pro-' meant 'for'; 'reform' originates from Latin, specifically the word 'reformare', where 're-' meant 'again' and 'formare' meant 'to form'.

'reform' changed from Latin 'reformare' into Old French/Anglo-Norman forms and Middle English 'reformen' before becoming the modern English 'reform'; the prefix 'pro-' was adopted from Latin into English compounds to indicate being 'for' something, producing the compound sense 'pro-reform'.

Initially the roots conveyed 'to form again' (reformare) and 'for' (pro-); over time the combined sense shifted to 'in favor of change or improvement' (supporting reforms).