'anti-monarchist' is formed from the prefix 'anti-' (from Greek anti- meaning 'against') combined with 'monarchist' (one who supports or concerns monarchy).

'monarch' comes from Greek 'monárchēs' (μόναρχης) via Latin and Old French into Middle English as 'monark'/'monarchy'; 'monarchist' developed from 'monarch' + suffix '-ist' in English; 'anti-' as a productive prefix from Greek entered English usage and combined with 'monarchist' to form 'anti-monarchist' (recorded in modern English usage from the 19th century onward).

Initially components meant 'against' (anti-) and 'single ruler' (monarch); the compound came to mean specifically 'a person opposed to monarchy as an institution' rather than simply 'against a particular ruler.'