'pro-parliamentarian' is a compound formed from the prefix 'pro-' (from Latin 'pro', meaning 'for' or 'in favor of') and 'parliamentarian' (relating to 'parliament').

'Parliament' comes from Old French 'parlement' ('a speaking, discussion'), from parler 'to speak'. 'Parliamentarian' developed in English to mean 'a supporter or member of a parliament' and the compound 'pro-parliamentarian' arose by combining 'pro-' + 'parliamentarian' to denote someone in favor of parliament.

Initially the components signified 'for' + 'parliament' (i.e., in favor of parliamentary discussion); over time the compound has been used specifically to label people or positions that support parliamentary authority or systems.