Sherlock had a theory that the human brain was like an attic.  Like an attic, it could become cluttered with useless things.  This rather odd theory came out during a conversation with Dr. Watson in which Sherlock mentioned having learned something and wanting to forget it as soon as possible so that his "attic" would have room for more useful knowledge.

I'm not sure how much faith we can put in his theory, but there are only 24 hours in each day.  An hour spent learning how to conjugate Farsi verbs is an hour we could have spent learning how to do something we are going to need today, tomorrow, and the next day.  An hour spent learning out dated or rarely used slang is an hour not spent learning what is current and heard/read often.  Or am I wrong?

Two really good sources of slang are comic books and comic strips in the newspapers.  The characters in these stories tend to speak very colloquially, making many references to culture, history, etc.  And you can rest assured that the slang is current.