In a manner befitting royalty; magnificently or thoroughly (often used for emphasis).
From Middle English roial, from Old French roial, from Latin regalis meaning 'of a king.' The adverbial form developed in English by the 14th century, with the intensifying sense ('thoroughly, completely') emerging in colloquial usage.
The word 'royally' has evolved from describing literal royal behavior to becoming a colorful intensifier in phrases like 'royally screwed up,' showing how language democratizes even the most aristocratic concepts. It's fascinating that we use the language of monarchy to emphasize the magnitude of both triumphs and disasters.
Complete word intelligence in one call. Free tier — 50 lookups/day.