https://en.wikipedia.org/wiki/Harald_Bluetooth
“The first documented appearance of Harald’s nickname “Bluetooth” (as blatan; Old Norse *blátǫnn) is in the Chronicon Roskildense (written ca. 1140), alongside the alternative nickname Clac Harald.[5] Clac Harald appears to be a confusion of Harald Bluetooth with the legendary or semi-legendary Harald Klak, son of Halfdan. The byname is given as Blachtent and explicitly glossed as “bluish or black tooth” (dens lividus vel niger) in a chronicle of the late 12th century, Wilhelmi abbatis regum Danorum genealogia.[6] The traditional explanation is that Harald must have had a conspicuous bad tooth that appeared “blue” (i.e. “black”, as blár “blue” meant “blue-black”, or “dark-coloured”). Another explanation, proposed by Scocozza (1997) is that he was called “blue thane” (or “dark thane”) in England (with Anglo-Saxon thegn corrupted to tan when the name came back into Old Norse).[7]”
