'pro-monarchical' originates from the Latin prefix 'pro' meaning 'for' combined with 'monarchical', which ultimately derives from Greek 'monarkhia' where 'monos' meant 'single' and 'arkhos' meant 'ruler'.

'monarchy' passed from Greek 'monarkhia' to Latin 'monarchia' and Old French 'monarchie', then into Middle English as 'monarchie'; the adjective 'monarchical' developed from this noun, and the modern adjective 'pro-monarchical' formed by prefixing Latin 'pro-' to 'monarchical'.

Initially it meant 'rule by one' (from the Greek origin), but over time it evolved to mean 'a system of government led by a monarch'; with the prefix 'pro-' it now specifically means 'supporting that system'.