"kingly, pertaining to a king," late 14c., from Old French regal "royal" (12c., Modern French réal) and directly from Latin regalis "royal, kingly; of or belonging to a king, worthy of a king," 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." Related: Regally.
"stables grouped around an open yard," 1630s, from Mewes, name of the royal stables at Charing Cross, built 1534 on the site of the former royal mews (attested from late 14c.), where the king's hawks were kept (see mew (n.2)). Extended by 1630s to "an alley or court in a large town on which stables are situated" and by 1805 to "street of former stables converted to human habitations."
1530s, "rights and powers of a king, royal privilege," from Latin regalia "royal things," noun use of neuter plural of regalis 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").
The meaning "decorations or insignia of an order" is recorded from 1670s, probably via the sense of "the emblems or insignia of royalty," e.g. the crown, scepter, etc. (1620s).