“Excalibur” was first used for King Arthur’s sword in the embellishment of the King Arthur legend by the French. Contrary to present-day myth, Excalibur was not the famous “Sword in the Stone” (which broke in battle), but a second sword acquired by the King
through the intercession of Myrddin (Merlin). Worried that Arthur would fall in battle, “Merlin took the King to a magical lake where a mysterious hand thrust itself up from the water, holding aloft a magnificent sword. It was the Lady of the Lake, offering Arthur a magic unbreakable blade, fashioned by an Avalonian elf smith, along with a scabbard which would protect him as long as he wore it . . .” http://www.britannia.com/history/arthur/excalibur.html
The Welsh name for King Arthur’s sword was ‘Caledfwlch’, which means ‘cleaving what is hard’. (from Celtic Culture: A Historical Encyclopedia). It later developed to become the Caliburn in the story by Geoffrey of Monmouth and finally the Frenchified Excalibur that we know today.
Caledfwlch first appears in Culhwch and Olwen, a Welsh tale dating perhaps to the 11th century (Geoffrey wrote the History of the Kings of Britain in the 12th century). From Celtic Culture: “Culhwch arrives at his kinsman Arthur’s court in Celliwig, seeking assistance in wooing the giant’s daughter, Olwen. Arthur’s speech to Culhwch includes a list of precious items that the young man may not request . . . ‘You shall have what your head and tongue may seek, as long as the wind dries, the rain wets, the sun moves, as far as land and sea encompass, except my ship and my mantle, Caledfwlch my sword, Rhongomiant my spear, Wyneb-Gwrthucher my shield, Carnwennan my knife, and Gwenhywfar my wife . . .'”
This is the full extent of the Welsh discussion of Excalibur, though swords held an almost ritualistic place in medieval/dark age culture. “Along with the spear, they were the ever-present symbol of the warrior class. The weapons of the warrior were not simply tools but deeply important spiritual symbols that stood for the ideal qualities of the warrior. The makers of weapons were highly regarded artisans, metalworkers whose skills were literally perceived as a gift of the gods. As a consequence, a sword was not simply an implement but an emblem of a higher spiritual truth, an emulation of the attributes of the gods.” http://www.netplaces.com/celtic-wisdom/the-tuatha-d-danann/the-sword-of-light.htm