(noun) The person who provides one with something; usually drugs, alcohol, pirated software, answers to a final, or something similiarly illicit.
For example: Who's your hook up for this awesome shit?

(noun) The stuff that your hook up provides you with.
As in: Hey, did you get the hook up yet?

(verb) To be the hook up and provide someone with a hook up.
For example: John's hook up hooked him up with the hook up.