'pro-United' is formed from the Latin prefix 'pro' (from Latin 'pro', meaning 'for') combined with the English word 'United' (the past participle of 'unite').

'United' derives from Old French/Anglo-Norman forms based on Latin 'unitus' (past participle of 'unire', from 'unus' meaning 'one'), passing into Middle English as 'united' and later combined with the prefix 'pro-' in modern English to form compounds like 'pro-United'.

Initially the elements meant 'for' (pro-) and 'made one' (united); over time the compound has come to mean 'in favor of the group called United' (typically used to indicate support).