1) What does "Zip it" mean "zip it, would you?"? Does it mean "shut up"?
Yes
zip up your mouth
2) What does "down" mean in "remember how you were down on me for wondering what a hen was"? Does it mean "laugh at"?
I don;t use this. Can only guess.
3) what does "a bucket of bull" mean in "you dumped me in a bucket of bull"?
I don;t use this. Can only guess.
4) what does "Deck" mean in "I want to deck somebody"?
Deck means to hit with the result that they fall down and hit the deck, the floor.
5) Does "Slug" mean "hit" in "I want to slug him"?
Yes.
punch.