A person who is "on the hook" will be overly infatuated with another person. The person who is the desired generally takes little notice (and often complete advantage) of the person who is on the hook.
Often times the person who is on the hook is a back-up.
Signs that you are on the hook:
1) giving foot rubs
2) making mixed tapes/cds/play lists
3) making chocolate cake
4) dropping everything at a moments notice to be with the other person.
Ted: "Lisa came over last night and I gave her a foot rub as we watched a move."
Marshall: "Are you guys dating now?"
Ted: "No, she is still with her boyfriend, she is just looking for the right time to break the news."
Marshall: "Dude, you are so on the hook."