USian is used to mean people who live in the US. Some people choose it because it's shorter, some because they think it's too US-centric to call people American when there are many other countries on two continents that could match that definition.

If you are a non-native English speaker, you probably want to avoid this term unless you are conscious that many readers will code you as "progressive left". It's not as strong as "Amerikan" (sometimes used to imply Americans are Nazis), but it has a similar register.

Here's an HN thread from a few years ago about it: https://news.ycombinator.com/item?id=9994159.