It's German for "pure white knight" or "true white knight".
Originally, it's a reference to a particular unit from the strategy RPG series Super Robot Wars, called the Weissritter [http://images.wikia.com/superrobotwars/images/3/35/Weiss_Ritter.jpg]. But, well, the Weissritter is a popular unit of a popular (at least in Japan) series, making taking it as a user name before someone else does unlikely.
"Weissritter", however, while indeed being German, and intentionally so, is grammatically inaccurate; "weiss" is the noun meaning "white", but as an adjective, "white" translates as "weisser". So, the correct way to say "white knight" in German would be "weisser ritter", as in, a knight that is white. "WeisserRitter", as it turns out, is a lot less common as a user name because not many people know that it's the correct spelling. Or care, for that matter; correct or not, it's technically still not the unit's name.
However, there's an upgraded form of the Weissritter known as the Rein Weissritter [http://images.wikia.com/superrobotwars/images/0/00/Rein_Weiss_Ritter.jpg], its name translated as above. I opted to take this as my user name, with the grammatically correct spelling, making it, arguably, the "pure" or "true" version of the name.
In short, it's a kind of a personal joke that sort of loses its amusement factor when I have to explain it like that. But hey, turns out it's a lot easier to get as my personal user name where ever I decide to use it.