SWIM stands for Someone Who Isn't Me and SWIY stands for Someone Who Isn't You. They're terms used by people who think that using them protects them against legal prosecution. First of all no prosecutor in the world is going to prosecute based off of just some forum posts you may or may not have made, you could just have been boasting. And if they do prosecute they have other proof and a forum post is the least of your worries. Second of all it doesn't protect you at all, any judge is going to look right through that
Just don't mention any ongoing or future significant criminal activity and you'll be fine. If you have any other acronyms you would like explained just ask. Welcome to bluelight!