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 Chrétien de Troyes' late 12th-century Old French "Perceval", Arthur's knight Gawain carries the sword "Escalibor" and it is stated, "for at his belt hung Escalibor, the finest sword that there was, which sliced through iron as through wood" (""Qu'il avoit cainte Escalibor, la meillor espee qui fust, qu'ele trenche fer come fust""). This statement was probably picked up by the author of the "Estoire Merlin", or Vulgate Merlin, where the author (who was fond of fanciful folk etymologies) asserts that Escalibor "is a Hebrew name which means in French 'cuts iron, steel, and wood'" (""c'est non Ebrieu qui dist en franchois trenche fer & achier et fust""; note that the word for "steel" here, achier, also means "blade" or "sword" and comes from medieval Latin "aciarium", a derivative of "acies" "sharp", so there is no direct connection with Latin "chalybs" in this etymology). It is from this fanciful etymological musing that Thomas Malory got the notion that Excalibur meant "cut steel" (""the name of it,' said the lady, 'is Excalibur, that is as moche to say, as Cut stele"").