Slang is as much an evolution of language as dialects are an evolution of language. What was the slang of yesteryear becomes the ordinary language of today. Before the western world's fascination with maths in the 1800s, double-negatives were common in the English language. In those early days, double-negative statements were seen as emphasising the negative, not negating it. The cultural fascination with maths changed that forever. If maths can alter the shape of language, imagine what the addition of other languages can do, as has happened in America. Words misheard are interpreted and given new meanings, repeated and misheard again. Sooner or later, new conventions spring up, some as slang.
American slang comes from immigrant communities with their own languages. It comes from subcultures' lexicons. It comes from the way children communicate on social media. It arrives in music, in film, in television shows and the typos in the chats of online video game players. American slang is less closed to the outside world, as with rhyming slang, but to the uninitiated, it makes little sense. American culture is inescapably spread all over the world, so you may have heard some of its slang, but do you really know the meaning of any of it? Test your knowledge, cowboy!