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.
Szczerbiec
Historical accounts related to the early history of the Polish coronation sword are scant and often mixed with legend. The earliest known use of the name "Szczerbiec" appeared in the "Chronicle of Greater Poland" at the turn of the 14th century. According to this source, the sword was given to King Boleslaus the Brave (reigned 992–1025) by an angel; Polish kings were supposed to always carry it in battle to triumph over their enemies. During Boleslaus's invasion of Kievan Rus', he hit it against the Golden Gate of Kiev while capturing the city. It was the notch that appeared on the edge of the blade which gave the sword its name. This account, written three centuries after the events it describes, is implausible not only because of the customary reference to the sword's supernatural origin (compare Excalibur), but also because Boleslaus's intervention in the Kievan succession crisis took place in 1018, or about 19 years before the actual construction of the Golden Gate in 1037.