Someone who is from the United States of
America, hence the "USA," in USAian.
Instead of referring to them as American, since
America is made up of two (or one continent depending on a country's continent model) continents: North
America, which ends at the isthmus of Panama. And South America, which starts in Punta Gallinas, Colombia.
Everyone from the Americas is American, just like everyone from any country in Europe is European. Therefore, the term "USAsian" is more specific to people from the United States.
Alex: Hi, where are you from?
John: I'm American!
Alex: Cool, me too! What part? I'm from Mexico.
John: Oh, I meant, I'm from
America, as in The United States.
Alyssa:
America is the entire continent, silly. I'm from
Canada, so we're ALL American. You're USAsian.