When having sex in the doggy style position, you pop your index finger in the brown hole, then pull it out and insert into the receivers mouth. Very similar to the "Fish Hook" which is the same thing without the "rust".
So last night i was with this girl i met at a bar. One thing led to another and we ended up doggy style at her place! I thought hey, Fish Hook!! But the finger is rusty? So i did it anyway. Hence Rusty Fish Hook!!
Jackson: The old huckelbuck failed me for the last time!
Josh: Huh?
Sir Issac Newton: According to my hypothesis, I'm clinically dead.
Pee-Wee Herman: What a heartless bitch!
Norwegian guy: Does anyone have some ice? I've got a Norwegian fish hook over here...