Lunfardo is the slang used in Buenos Aires, which on occasion takes over more of the conversation than actual Spanish. Some say this slang was created as a mixing of the different cultures that settled here, or as a way to speak in code so police wouldn't understand. Either way, it still has me scratching my head sometimes, even though I've gotten down some of the basics.
Some of the most common words that many foreigners will either know already or learn right away are, "che," "boludo," or "che boludo." These words can mean a number of things, but depending on the situation, che is more like "dude", or "hey." Boludo can be something you call a friend or something you call an enemy, and as I've already made the mistake several times, you need to be comfortable with someone before you drop the boludo. I even have a little book on lunfardo-English called, "Che Boludo!"
It's something of a joke, but you really do need to learn a bit. A bad word in another language will never mean the same thing to you as one in your own language, and that's why I say boludo too much. I just don't feel it. To me, I hear people saying the word and think, "Okay, I get it. I want to say it too and look like a local." But I don't hear it in the same way, and I don't hear the undertones. I had just met a kid on my floor and said boludo. He wasn't happy, and I was the boludo.
Think of some of the worst words in the English language, and how you would instinctively know when it is socially acceptable to say them. Now, imagine that you have a foreign friend who hears you say one of these words. The next day they drop the word in randomly at a totally inappropriate time. Under normal circumstances you might be offended, but knowing that they don't fully grasp the word, you have to just smile and explain that they shouldn't say that. This scene is basically my life down here.
I'm learning on a moment by moment basis, and even then it takes time to get the meaning of the word behind the word. It's not just Americans or Britons that make these mistakes though. People from Buenos Aires talk differently, and even Spaniards or people from the south of Argentina are known to mock the way porteños speak. Like seeing an actor butcher a Boston accent, it makes you cringe.
Wherever I've gone I've tried to learn the local lingo. In Spain that meant saying, "Tío, joé que caló!" In Ecuador I adopted, "A cha chay," and "chévere." Even after just ten days or so in Chile I started using, "po" and "huevón." So no surprise that down in Argentina, trying to get to know this culture as best as I can, I'm going to mimic and do as the locals do. I just need to make sure I'm not saying the wrong thing.
I want so badly to be perfectly fluent in Spanish, and it's obvious to me that this is a goal that will never be accomplished. But there has to be a balance between sounding like an idiot and getting away with mistakes to sounding too good to make blatant mistakes like calling someone you just met an asshole, all with a smile on your face. Oddly enough, it's kind of a challenge.