We couldn’t cover names from Arthuriana without covering the name of arguably the most important character, bar Arthur himself: Merlin. This character owes his existence to Geoffrey of Monmouth, who named him Merlinus Ambrosius (Welsh: Myrddin Emrys). We’ll devote two posts to his name, considering both the Latin and Welsh forms of the first name in this post and then Latin and Welsh forms of the second name in the next post.
The root of Merlin’s given name lies in the city of Carmarthen, which was originally a 2nd C Roman fort named Maridunum or Moridunum ‘fort by the sea’. Later forms of the name were influenced by Old Welsh *Morddin, which developed into merddin and myrddin, and then Welsh caer ‘fort, fortified settlement’ was added. In the 12th C (Geoffrey was writing around 1136), the city’s name was recorded variously as Caermerthin, Cairmerdin, and Kaermerdyn. Geoffrey took these names, and interpreted the second element as a personal name, Merdin or Myrddin.  On this point, Hutson delightfully comments:
The fact that Geoffrey connects Merlin with Caermarthen is evidence that Geoffrey is probably eponymizing again .
He goes on to explain that
The shift from Merdinus to Merlinus has been explained by Lot as an attempt to avoid an unpleasant association with the French merde, and this seems a good reason .
There is little to no evidence that Myrddin was ever used as a personal name as a result of Arthurian influence, before modern times. Merlin, on the other hand, was sporadically used in England in the 13th and 14th C, with examples such as Ralph filius Merlin 1202, Jon Merlini c1210, Henry Merling 1327, and John Merlyn 1347 . By the 14th C, the name had also made it to the Netherlands, in the spelling Merliin . The name can also be found in Germany, but there it is not entirely clear that the Arthurian character is the root. When used by women, Merlin can be a diminutive of Margaret (see this post for a discussion of the suffix.), and it can also be a derivative of Middle High German merle ‘thrush’.
Next post, we’ll pick up Ambrosius/Emrys.
 Hywel Wyn Owen & Richard Morgan, Dictionary of the Place-Names of Wales, (Gomer, 2007): s.n. Carmarthen.
 Hutson, Arthur E., British Personal Names in the Historia Regum Britanniae (Berkeley: University of California Press, 1940), p. 59.
 Reaney & Wilson s.n. Merlin.
 De rekeningen van de grafelijkheid van Holland uit de Beierse periode, Serie I: De rekeningen van de tresorier en de dijkgraaf van de Grote Waard, Deel: 1393-1396. (Den Haag: Instituut voor Nederlandse Geschiedenis, 1997).
 Bahlow, Hans, Dictionary of German Names, tr. Edda Gentry (German-American Cultural Society, 1994 ISBN: 0924119357), s.n. Merl(e).