Open Backpack

Meaning that you can see someone's butt crack or underwear slightly as a person is bending over similar to the zipper of a backpack being slightly open.

(Emily bends down to grab books) Me: Hey Emily you've got an open backpack
Emily: Did you notice my new Victoria's Secret thong?
Me: No I was just looking at your crack.
Emily: You're an asshole.
Me: LOL get it?

Your backpack's open.

You're walking around completely clueless while making a fool of yourself.

Your backpack's open. Your making a fool of yourself.