Means (male) masturbation. Propably due the following joke:
Two men walk into a toilet.
While man #1 starts pissing, man #2 starts counting numbers aloud.
1, 2, 3, 4..., 5, 6, 7.
Man #1: What were those numbers all about?
Man #2: You see boy, I like to keep things organized.
When I say 1, I
unzip the zipper.
When I say 2, I take my dick out.
When I say 3, I pull back the foreskin.
When I say 4, I piss.
When I say 5, I pull forth the foreskin. When I say 6, I put my dick back in my underwear.
And when I say 7, I zip the zipper.
But the next day, the two men go back to the bathroom, and suddenly the man starts yelling in the bathroom 1, 2, 3,5,3,5,3,5,3,5,3,5,3,5!!!