royalyoudaoicibaDictYouDict[royal 词源字典]
royal: [14] Royal and regal are ultimately the same word. Both go back to the Latin adjective rēgālis, a derivative of rēx ‘king’. But whereas regal was probably borrowed direct from Latin, royal was acquired via Old French, where rēgālis became roial.
=> regal[royal etymology, royal origin, 英语词源]
royal (adj.)youdaoicibaDictYouDict
mid-13c., "fit for a king;" late 14c., "pertaining to a king," from Old French roial "royal, regal; splendid, magnificent" (12c., Modern French royal), from Latin regalis "of a king, kingly, royal, regal," from rex (genitive regis) "king" (see rex). Meaning "thorough, total" attested from 1940s; that of "splendid, first-rate" from 1853.

Battle royal (1670s) preserves the French custom of putting the adjective after the noun (as in attorney general); the sense of the adjective here is "on a grand scale" (compare pair-royal "three of a kind in cards or dice," c. 1600). The Royal Oak was a tree in Boscobel in Shropshire in which Charles II hid himself during flight after the Battle of Worcester in 1651. Sprigs of oak were worn to commemorate his restoration in 1660.
royal (n.)youdaoicibaDictYouDict
"royal person," c. 1400, from royal (adj.). Specifically "member of the royal family" from 1774.