Form of the Welsh name Myrddin
(meaning "sea fortress") used by Geoffrey of Monmouth in his 12th-century Arthurian tales. Writing in Latin, he likely chose the form Merlinus
in order to prevent associations with French merde
Geoffrey based parts of Merlin's character on Myrddin Wyllt, a semi-legendary madman and prophet who lived in the Caledonian Forest. Other parts of his life were based on that of the historical 5th-century Romano-British military leader Ambrosius Aurelianus. In Geoffrey's version of the tales and later embellishments Merlin is a wizard and counselor for King Arthur.