royal (adj.)

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," from PIE root *reg- "move in a straight line," with derivatives meaning "to direct in a straight line," thus "to lead, rule."

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.)

"royal person," c. 1400, from royal (adj.). Specifically "member of the royal family" from 1774.

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Definitions of royal from WordNet
royal (adj.)
of or relating to or indicative of or issued or performed by a king or queen or other monarch;
a royal visit
the royal party
by royal decree
the royal crest
royal (adj.)
established or chartered or authorized by royalty;
royal (adj.)
being of the rank of a monarch;
of royal ancestry
princes of the blood royal
royal (adj.)
belonging to or befitting a supreme ruler;
the royal carriage of a stag's head
treated with royal acclaim
Synonyms: imperial / majestic / purple / regal
royal (adj.)
invested with royal power as symbolized by a crown;
the royal (or crowned) heads of Europe
royal (n.)
a sail set next above the topgallant on a royal mast;
royal (n.)
stag with antlers of 12 or more branches;
Synonyms: royal stag