'royalistly' originates from Modern English, specifically formed from the adjective 'royalist' plus the adverbial suffix '-ly', where 'royalist' comes from French 'royaliste' and 'royaliste' from 'royal' (Old French 'roial') ultimately from Latin 'regalis'.

'royalistly' changed from the adjective/noun 'royalist' (from French 'royaliste'), which developed from Old French 'roial'/'royal' and Latin 'regalis', and the suffix '-ly' was later attached in Modern English to form the adverb 'royalistly'.

Initially the roots related to 'kingly' or 'royal' (pertaining to a king); over time the derived terms came to mean 'supporting the monarchy' and 'in the manner of those who support the monarchy' as in the modern adverbial use.