'pro-monarchy' is formed from the combining element 'pro-' (from Latin 'pro', meaning 'for') attached to 'monarchy' (from Greek 'monarkhia' where 'monos' meant 'one' and 'arkhein' meant 'to rule').

'monarchy' entered English via Latin and Old French from Greek 'monarkhia' and became Middle English 'monarchie'; the prefix 'pro-' (Latin 'pro') has long been used in English to mean 'in favor of' and has been attached in modern usage to form compounds like 'pro-monarchy'.

Initially, 'monarchy' meant 'rule by one' (from Greek), and 'pro-' meant 'for'; combined, 'pro-monarchy' developed to mean 'in favor of the institution of monarchy' rather than describing the structure itself.