What do you think about me  telling a guy to go back to where he came from?
Why would you tell someone to go back somewhere?
Asian foreigners should go back to Thailand or wherever they came from.
Telling someone to go back where they came from is a racial insult, and it is mainly used to target immigrants and ethnic groups whose members are falsely considered to be immigrants. You might find out where they actually came from before you speak like this to someone.
Why are you getting so serious about it?
Judging someone based on their race is a serious matter. You should consider where your ancestors came from before saying that to another person.