1) Where a girl is so excited that when she give a guy a BJ, she doesn't unbutton the top button, she leaves it buttoned and pulls down the zipper only.
2) Also, if a girl is giving you a BJ in a public place.
1) "Dam dude last night, she wanted to suck my cock so bad that she gave me a zipper job!" "WTF, nice!!"
2) "Dont look over there kids, theres a girl giving a guy a zipper job!"