* Origin: The last name Thiessen is primarily of German origin. It is a variant spelling of "Thießen", which is a place name meaning "house by the thistles".
* Distribution: Thiessen is most common in Germany and Canada, likely due to German immigration. It's also found in the United States and Australia.
However, without additional details, it's impossible to say with certainty what nationality someone with the last name Thiessen might be.