Hello Chola and welcome to the forums. Slang is singular and uncountable - it doesn't have a plural form. If you type American Slang into your browser, you will find lots of sites, but here are some to start you off:

Hi, onlyme, and welcome to the forum.

You can read all about it here, guest http://www.bbc.co.uk/worldservice/learningenglish/grammar/learnit/learnitv316.shtml

It should be " fourth most", guest.

That's how I would use it, Nona.

Multiple Independently targetable Re-entry Vehicle http://en.wikipedia.org/wiki/Multiple_independently_targetable_reentry_vehicle

it is a sarcastic comment, Antonia, like "and pigs can fly".

Here it is, Antonia. http://en.wikipedia.org/wiki/Rose_is_a_rose_is_a_rose_is_a_rose.

Now don't get me all worked up and then let me down, Pieanne!