'pro-federal' originates from Latin elements: the prefix 'pro-' (Latin 'pro') meaning 'for', and 'federal' ultimately from Latin 'foedus' (via Medieval Latin 'foedalis'), where 'foedus' meant 'treaty' or 'pact'.

'federal' changed from Latin 'foedus' into Medieval Latin 'foedalis', passed into French/late Latin usage and Middle English as 'federal'; the modern compound 'pro-federal' is formed by combining Latin-derived prefix 'pro-' with English 'federal'.

Initially, elements related to 'foedus' conveyed 'treaty' or 'league'; over time 'federal' came to denote a system of government based on a union of states under a central authority. Thus 'pro-federal' evolved to mean 'in favor of a federal system' or 'supportive of central federal authority'.