Origins
The word "regal" traces directly to the Latin vocabulary of kingship, connecting modern English to the oldest Indo-European concepts of authority and rule. It entered English around 1374 from Old French "regal" (royal), from Latin "rēgālis" (of or belonging to a king), from "rēx" (genitive "rēgis," king), from "regere" (to rule, to direct, to keep straight), from Proto-Indo-European *h₃reǵ- (to straighten, to direct).
The relationship between "regal" and "royal" is one of English's most instructive doublets — pairs of words derived from the same source but entering the language at different times or through different channels. Both come from Latin "rēgālis." "Royal" entered English earlier, through the heavily altered Old French form "roial" or "real" (where Latin -gāl- became French -al-/oi-). "Regal" arrived later, through a more learned channel that preserved the Latin form more faithfully. The result is two English words with the same etymological meaning but different connotations: "royal" is the everyday word (royal family, royal decree), while "regal" is more literary and emphasizes grandeur or magnificence (regal bearing, regal splendor).
This pattern of learned/popular doublets recurs throughout English. "Fragile" (learned Latin) and "frail" (popular French) both come from Latin "fragilis." "Legal" and "loyal" both come from Latin "legālis." "Hospital" and "hotel" both come from Latin "hospitāle." In each case, the learned form is closer to Latin and often more formal.
Proto-Indo-European Roots
The PIE root *h₃reǵ- (to straighten, to direct, to rule) is among the most consequential in the Indo-European family. From it descended: Latin "rēx" (king), giving English "regal," "royal," "reign," "regicide," and the name "Rex"; Latin "regere" (to rule), giving "regulate," "regime," "regent," "region," and "direct"; Latin "rēgula" (straight stick, rule), giving "regular" and "rule" itself; Sanskrit "rājan" (king), appearing in "rajah" and "maharaja"; Irish "rí" (king); and, through Germanic, Old English "riht" (right, proper, straight), giving modern English "right."
The connection between straightness and kingship is deeply embedded in Indo-European thought. To rule was to keep things straight — to maintain order, to set things right. A king (rēx) was one who directed affairs along the proper path. This metaphor — authority as straightening, justice as correctness — persists in modern English: we speak of "rectifying" errors (making them straight), "directing" operations (keeping them on course), and "correcting" behavior (making it right).
In English usage, "regal" carries stronger connotations of visual majesty and dignified bearing than "royal," which is more institutional and practical. A queen has "royal" duties but a "regal" presence. A palace is a "royal" residence where guests receive "regal" treatment. The distinction is not absolute, but the pattern holds: "royal" attaches to official status, "regal" to aesthetic impression.
Latin Roots
The Latin noun "rēgālia" (royal privileges, royal insignia) gave English "regalia" — the ceremonial objects of royalty, including crown, scepter, and orb. By extension, "regalia" now means any elaborate or special clothing, as in academic regalia (caps and gowns).
The eagle as a "regal" bird reflects centuries of royal symbolism. The Roman eagle (aquila) was the standard of the legions. The double-headed eagle became the symbol of the Holy Roman Empire, the Russian Empire, and many other states. The bald eagle became America's national symbol in 1782 — and is routinely described as "regal" in appearance, connecting the republic to the very vocabulary of monarchy it had rejected.
From the PIE concept of straightening and directing, through Roman kings and medieval French royalty, to modern English adjective of admiring description, "regal" preserves one of the oldest metaphors in Western civilization: that to be kingly is to be upright, straight, and commanding.