There is no evidence that the name “Bluetooth” was used during Harald’s life time. The first written mention of “Bluetooth” is found in the Roskilde Chronicle from c1140. However, there were a number of contemporary Harald’s who all had nicknames (Greycloak, Guld, Strut), and it was a Viking tradition. Therefore, there is also no reason to think that it was not used by contemporaries.
There are a number of hypothesis as to the origin of the nickname (listed below in no particular order)
- Had dark hair and/or dark complexion – The name “Blåtand” was likely taken from two Old Danish words, blå meaning “dark skinned” and tan meaning “great man.”
- Had a damaged or dead tooth that had turned blackish-blue – dental care was not very advanced in the Viking Age. With a lot of fighting, then getting hit in the face is not unlikely.
- Had groove etched in tooth/teeth with a dark or blue dye applied – this was a Viking tradition, of which the actual context and meaning is as yet unknown. However, has possibly been related to warfare or mating ritual. If widespread, then conspicuous that other Viking leaders would not be referred to in similar fashion by chroniclers or the like.
- “Blåtan” came about as a corruption of the English “Thegn” og “Thane” (meaning “chief”) – Since blue can also mean “dark”, then “the blue Thegn” (meaning dark chieftain) may have become Bluetooth when name was translated back into Old Norse.
- Harald wore blue cloth/leather – Another possible explanation is proposed by the curator at the Royal Jelling Hans Ole Mathiesen. The color blue was the most expensive dye (associated with kings), and so by wearing blue Harald underlined his royal lineage. The “tan” here meaning “hide” or “leather”, so essentially “Blue leather” or “Blue hide”.
- Had a liking for blueberries – this hypothesis is based on blueberries staining teeth a dark blue/black-ish color. Blueberries are known berries of the Viking Age in Denmark, so that part checks out. Yet, if blueberries were widely eating by resident Vikings, then it goes to reason that a large number would have blue teeth. And so, it would not be a remarkable feature of the leader alone.
On the official Danish Royal family homepage (kongehuset.dk) it merely states that “Bluetooth” likely derived from a discolored tooth. However, no reason is ventured.