'pro-saloon' is a compound formed in Modern English from the prefix 'pro-' (from Latin 'pro' meaning 'for' or 'in favor of') plus the noun 'saloon' (from French 'salon', ultimately from Italian 'salone').

'saloon' entered English from French 'salon' (meaning a large room or reception hall) and in the 19th century came to be used for public drinking establishments; combined with the productive English prefix 'pro-' it yielded the compound 'pro-saloon' to label support for such establishments.

Initially it literally meant 'for the saloon' (i.e., in favor of the establishment itself); over time it has been used more broadly to mean opposition to temperance/prohibition and support for commercial drinking venues.