Arthur is a male given name possibly of Brythonic Celtic origin. Another theory is that the name is from Latin origin, derived from the Roman clan Artorius who lived in Roman Britain for centuries. Its popularity derives from it being the name of the legendary hero King Arthur. It may derive from the Celtic Artos meaning “Bear”. 
 A common spelling variant used in many Slavic, Romance, and Germanic languages is Artur. In Spanish and Italian it is Arturo.
 The earliest attestation of the name Arthur is in the early 9th century Welsh-Latin text Historia Brittonum, where it refers to a circa 5th century Romano-British general who fought against the invading Irish and Picts, and who later gave rise to the famous King Arthur of medieval legend and literature. A possible earlier mention of the same man is to be found in the epic Welsh poem Y Gododdin by Aneirin, which some scholars assign to the late 6th century, though this is still a matter of debate and the poem only survives in a late 13th century manuscript entitled the Book of Aneirin. A 9th-century Breton landowner named Arthur witnessed several charters collected in the Cartulary of Redon.
 The Irish borrowed the name by the late 6th century (either from an early Archaic Welsh or Cumbric form Artur), producing Old Irish Artúr (Latinized as Arturius by Adomnán in his Life of St. Columba, written circa 697–700), The earliest historically attested bearer of the name is a son or grandson of Áedán mac Gabráin (died 609).
 The exact origins of the name Arthur remains a matter of debate. Some claim that it derives from the Roman nomen gentile (family name) Artorius. Artorius itself is of obscure and contested etymology; some have proposed a Messapic or Etruscan origin. Nevertheless, according to etymologist Kemp Malone "the theory that derives Arthur from Artorius is left unsupported by any evidence worthy of the name". 
 According to the linguist and Celticist Stefan Zimmer, it is possible that Artorius has a Celtic origin, being a Latinization of the hypothetical name *Artorījos, derived from the patronym *Arto-rīg-ios, meaning "Son of the Bear/Warrior-King". *Arto-rīg-ios is unattested, but the root, *arto-rīg, "bear/warrior-king", is the source of the Old Irish personal name Artrí, while the similar *Arto-maglos, "bear-prince", produced names in several Brittonic languages. According to Zimmer's etymology, the Celtic short compositional vowel -o- was lengthened and the long -ī- in the second element of the compound -rījos was shortened by Latin speakers, under the influence of Latin agent nouns ending in -tōr (and their derivatives in -tōrius). Some scholars have noted that the legendary King Arthur's name only appears as Arthur, Arthurus, or Arturus in early Latin Arthurian texts, never as Artōrius (although the Classical Latin Artōrius became Arturius in some Vulgar Latin dialects). However, this may not say anything about the origin of the name Arthur, as Artōrius would regularly become Art(h)ur when borrowed into Welsh.
 The commonly proposed derivation from Welsh arth "bear" + (g)wr "man" (earlier *Arto-uiros in Brittonic) is not possible for phonological and orthographic reasons; notably that a Brittonic compound name *Arto-uiros should produce Old Welsh *Artgur (where -u- represents the short vowel /u/) and Middle/Modern Welsh *Arthwr and not Arthur (where -u- is a long vowel /ʉː/) In Welsh poetry the name is always spelled Arthur and is exclusively rhymed with words ending in -ur—never words ending in -wr—which confirms that the second element cannot be [g]wr "man").
 An alternative theory, which has gained limited acceptance among scholars, derives the name Arthur from the Latin Arcturus (the brightest star in the constellation Boötes, near Ursa Major or the Great Bear), which is the latinisation of the Greek Ἀρκτοῦρος (Arktouros) and means Bear Guardian from ἄρκτος (arktos "bear") and οὖρος (ouros "watcher/guardian"). This form, Arcturus would have become Art(h)ur when borrowed into Welsh, and its brightness and position in the sky led people to regard it as the "guardian of the bear" and the "leader" of the other stars in Boötes.
 Avestan aṣ̌a/arta and its Vedic equivalent ṛtá both derive from Proto-Indo-Iranian *ṛtá- "truth", which in turn continues Proto-Indo-European *h2r-to- "properly joined, right, true", from the root *h2ar. The word is attested in Old Persian as arta.
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