You might have some sense of what foreign means or implies. When you encounter that word, you might think of someone from a different country or something that is the opposite of native. The logic applies to how “foreign” is used in rap.
In rap, when you say “foreign,” you are usually referring to an imported or foreign model of car. These are often expensive and luxurious supercars. When you have a foreign, you are someone big. You might hear it being used similar to these:
“Big J got some foreign after hitting it big with his single.”
“Some foundations let kids ride foreigns to make their dreams come true.”