Don’t need my own ideas of how they were called Wong Fu. Since it was a very common question, they have answered it many times.
Direct from their old homepage:
What does “Wong Fu” mean?
Wong fu is a nick name Philip had since he was in middle school. In all his video projects in high school he would just called them “wong fu productions”. Coming to college and making the early music videos on the site, he used the same name, and since then it’s just stayed that way. (yes his last name is spelled wANG…but it’s pronounced wONG. They’re the same word)