Zipper Bumping

male becomes erect enough that his penis is pressed against the zipper of his pants.

Yo Ming! you see those two asian school girls making out on the bus, got me straight up zipper bumping....ya digggggg?

head rash

The burning sensation that happens to the head of your penis while pressed up against the zipper of your pants for a long period of time

Judy, I got some serious head rash from our dry sex-capade last night