'pro-liberal' originates from Latin and English combining elements: specifically the prefix 'pro-' from Latin 'pro' meaning 'for', and the adjective 'liberal' from Latin 'liberalis' (from 'liber') meaning 'free'.

'liberal' changed from Latin 'liberalis' into Old French 'liberal' and then into Middle English as 'liberal'; the prefix 'pro-' (from Latin) has long been used in Late Latin and English to form compounds, and the modern English compound 'pro-liberal' arose by combining these elements to mean 'for liberalism'.

Initially, 'liberal' meant 'of or befitting a free person; generous' (from Latin), but over time it evolved into a political sense of 'supporting individual rights, reform, and progressive policies'; 'pro-liberal' therefore now commonly means 'supportive of these political positions'.