Excalibur
Geoffrey of Monmouth, in his "Historia Regum Britanniae" ("The History of the Kings of Britain", c. 1136), Latinised the name of Arthur's sword as "Caliburnus" (potentially influenced by the Medieval Latin spelling "calibs" of Classical Latin "chalybs", from Greek "chályps" ["χάλυψ"] "steel"). Most Celticists consider Geoffrey's "Caliburnus" to be derivative of a lost Old Welsh text in which "bwlch" (Old Welsh "bulc[h]") had not yet been lenited to "fwlch" (Middle Welsh "vwlch" or "uwlch"). In the late 15th/early 16th-century Middle Cornish play "Beunans Ke", Arthur's sword is called "Calesvol", which is etymologically an exact Middle Cornish cognate of the Welsh "Caledfwlch". It is unclear if the name was borrowed from the Welsh (if so, it must have been an early loan, for phonological reasons), or represents an early, pan-Brittonic traditional name for Arthur's sword.
Excalibur
In Welsh legends, Arthur's sword is known as "Caledfwlch". In "Culhwch and Olwen", it is one of Arthur's most valuable possessions and is used by Arthur's warrior Llenlleawg the Irishman to kill the Irish king Diwrnach while stealing his magical cauldron. Irish mythology mentions a weapon "Caladbolg", the sword of Fergus mac Róich, which was also known for its incredible power and was carried by some of Ireland's greatest heroes. The name, which can also mean "hard cleft" in Irish, appears in the plural, "caladbuilc", as a generic term for "great swords" in "Togail Troi" ("The Destruction of Troy"), the 10th-century Irish translation of the classical tale.