He was renowned throughout Denmark and Norway for his ability to pair with another device.

Oh nice I was just at the Jelling monumenterne like 3 days ago, very cool place

The first documented appearance of Harald’s nickname “Bluetooth” (as blatan; Old Norse *blátǫnn) is in the Chronicon Roskildense (written c. 1140), alongside the alternative nickname Clac Harald. Clac Harald appears to be a conflation 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. The traditional explanation[according to whom?] 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 tanwhen the name came back into Old Norse).