Red5 and Natalie27 : Thanks a lot for your replies and useful links. Thanks to you, I have now the right tools to make my research of definitions easier.
Nahualli: I agree with the most part of your obsvervations but I've never pretend that my personal method to practice english was the best way in order to make progress in english, or even a good one . Likewise, I've never meant that the hooks in the rap songs were profound or that this kind of lyrics were usually and widely used in the spoken language. As you said, it's an entertainment and I think that it's the strength of this kind of training. I don't really have the time to do grammar exercises, and for sure, I won't be able to pass the TOEFL or TOEIC exam watching some Jay-Z videos.
My main goal isn't to go in a fast food and speak like a rap singer saying: 'Yo shawty (thanks for definition Natalie27 :wink: ), I'm gonna take...' However, I've really made some progress in my listening comprehension: it seems easier when you listen a 'normal' conversation, with a 'normal' rhythm and with 'normal' words. But of course it's only my opinion. I just want to know the overall meaning of what I'm listening. Moreover, if I want to hum the lyrics of one song in order to practice my pronunciation I need to understand what I'm singing
Have you seen the movie 'The usual suspects' for instance? I've heard a lot of words like 'bullshit', 'bloody...', funking...' in this one. If I just want to know the difference between these expressions, it's only to understand the dialogue, and not to walk down the street and insult the first person I'll meet with the vocabulary I've just heard in the movie.
It's the same with the rap songs: it's funny (for me) but I don't take these lyrics at face value and I won't pretend understand english if I can understand even one of these songs. Sorry but I've never wanted to shock someone with this topic.