'pro-socialist' originates as a compound of Latin-derived 'pro' and the English word 'socialist'. 'pro' originates from Latin 'pro', where 'pro' meant 'for, in favor of'. 'socialist' comes from 'social' + suffix '-ist' (see below).

'socialist' entered English in the 19th century from French 'socialiste', itself built from Latin-based 'socialis' ('of companionship, of society') and the modern suffix '-ist'; the prefix 'pro-' (Latin 'pro') was later attached in English to form the compound 'pro-socialist' meaning 'for socialism'.

Initially, elements meant 'for' (pro-) and 'relating to society' (social-); over time the compound came to mean specifically 'in favor of socialism' rather than a general social orientation.