'anti-royalist' originates from the prefix 'anti-' (from Greek 'anti', meaning 'against') combined with 'royalist' (derived from Old French 'roial' / 'royal', ultimately from Latin 'regalis').

'anti-royalist' formed in modern English by joining 'anti-' + 'royal' + the agentive suffix '-ist'. 'Royal' comes from Old French 'roial' (from Latin 'regalis'), and the suffix '-ist' (from French/Latin) denotes a person who supports or opposes a position.

Initially it meant 'against the king or monarchy'; over time it has remained essentially the same, referring to those who oppose monarchy or royal authority.