You won’t find them in Italian books, but you’ll hear them all over the streets. Understanding them will give you a huge boost in comprehension. Using them will make you much more fun to talk to.
A really nice article about common words you will hear in Italy but won’t learn on textbooks. Italian slang is wide and varied, depending on city, region, even the person you’re talking to. It’s interesting to know more about these expressions because some don’t have an English parallel, others may even sound offending if you don’t know their meaning in the source language (check the comments in the article!) and they’re generally very fun to use.
I only have one thing to comment about (besides pointing out that “Fa cagare!” is more used than “Mi fa cagare!” because we like to cut words whenever we can), and it’s entry #10. It’s only used in Florence, so you won’t hear it anywhere else and if you use it, say, in Milan, people won’t understand you. I would have rather left that spot for the universal “Boh” (“bones”), which you will hear very, very often. There is no translation, but I can tell you it’s used to express doubt and uncertainty. Basically “I don’t know” or “I have no idea”, but shorter and more effective: “Are you hungry?” “Boh!” (I’m not sure, I can’t tell), or “When’s the next bus gonna be here?” “Boh!” (I have no idea and why would I even know that?). Tone is very important to deliver a good “Boh”. It sounds like you’re asking a question, but not really.
Have any of you heard of some Italian expressions you didn’t understand during your travels? Or maybe in a movie, or on the Internet. You have some slang expressions from your own language you want to check for an Italian parallel? Ask away!