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.