Regal is the Latin top of the ladder kingly, royal, regal. English took it in the late 1300s from Old French regal and directly from Latin regalis, "of a king," from rex, "king."
The interesting part is that royal and regal are the same Latin word, borrowed twice. Royal came through Old French roial, which had worn regalis down over centuries of French speech; regal was taken straight off the Latin page at almost the same period. So English acquired both the eroded French form and the pristine Latin one, and made them do different work — royal is the ordinary official word, regal the self-conscious literary one. Underneath rex is an Indo-European root meaning "to move in a straight line," hence to direct, to lead, to rule, which also gives right, regular, direct and reign.
regal (adj.) "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 . also from late 14c.
regal came in through Latin, and its spelling follows that family's conventions.
On the register ladder
English kept all three words for this idea, one from each layer, and the rung is itself the cue: the native word is blunt and concrete, the French one polite, the Latin one formal or technical.
| Old English | French | Latin |
|---|---|---|
| kingly | royal | regal |