It's possible that it's a slang term or a word from another language, but without more context, it's impossible to know for sure what it means.
Could you provide more information about where you encountered this term? Perhaps you can tell me:
* The full sentence or phrase it was used in.
* The context of the conversation.
* Where you heard or read it (e.g., online, in a book, etc.).
With more information, I might be able to help you figure out what "j'swei" means.