North American urban slang term meaning either "lots of," or "very." I rather like this usage because it may refer to either a quantity or a quality, and in neither case needs a preposition to be correct. It's meaning has to be gleaned from context, but in casual conversation this is always easy. AFAIK, it is the only word in English that has the flexibility to both quantify and qualify.

To illustrate, both of these sentences are correct:

I can't come to the party, I have mad cars waiting at the shop.
I'm mad hungry, let's order a pizza.

And so forth.