Zip slip

The act of your dick unintentionally falling out of the front flap in your underwear. NOT to be confused with the more ambiguous dick slip

Brad: Ow, fuck! Yo Chad, wait up!
Chad: What’s wrong bro, can’t keep up?
Brad: Nah bro, I had a zip slip!
Chad: Ouch, man! Hope you didn’t chafe against the zipper too bad!
Brad: All good, dude. This one was far from my worst.

Zip n' Slip

Unzipping your pants and proceeding to fuck without removing clothing. Similar in meaning to a quickie.

Example 1:
Bro 1: "How was the party?"
Bro 2: "It was awesome man, my girlfriend gave me the look and led me into the bathroom for the ol' zip n' slip!"
Bro 1: "Niiiice!"

Example 2:
Girl 1: "How was the movies?"
Girl 2: "It was spicy, my boy and I got some sex in with the ol' zip n' slip!"
Girl 1: "Niiiice!"