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.