Most predominantly used in the greater New York area, "mad" is an appropriate replacement for Northern California's "hella" and Boston's "wicked." In the common vernacular, it translates into "a lot" or "extremely." Can be used almost interchangeably with any of the above listed words.

For the most part, it means angry.

Wikipedia says:
A MAD Movie is a Japanese fan-made video, much like an anime music video (AMV), that generally originate from the Japanese website Nico Nico Douga. MAD can also describe the Japanese AMV community, although they can be anything from audio clips, edited pictures, to wholly original creations. MADs do not necessarily even need to be related to anime, though the more popular ones typically are. The term MAD (occasionally M@D) is thought to be a reference to the name of one of the original MADs, rather than a particular acronym for anything.

1: really or extremely
2: alot

(adj.) an adjective used to enhance a noun.

a person youd love to have living under your bed, good company at the very worst of times. someone who undertands what goes thru my head, and knows more about me than i do!

New York and New Jersey's way of saying very, it is similar to hella, wicked.

1. crazy, insane, demented, nuts, deranged, out of one's mind, bonkers, lost one's marbles.

2. angry (usually limited to the United States)

3. extremely, very