a gradeschool prank usually you would write:
on your hand, then ask if they wanted to join the pen15 club. it worked best if you had a bunch of people do it. then you would get them to go through the secret ceramony where you would write "pen15" in the so that it looked like you wrote "Penis" and you would point and laugh. best done with sharpie marker.
dude: Hey I joined the pen 15 club!
chick: Pen 15 club my ass you have penis on your hand, get the fuck away from me!
A club in most middle schools around the country. Consisting of mostly prepubescent boys.
Each member receives a "club card" with their name and "Pen15
Club Member" on the card.
When the new member receives the club card then realizes that he/she has just joined the penis club.
Joe: Billy do you want to join the Pen Fifteen Club?
Billy: YES! I will conform to you all because I want to be cool and popular!
Joe (while snickering): Here is your membership card!
Billy: You wrote Pen15 Club! that is the same as the Penis Club! HAHA, OMFG, LMAFO, LOLZ!
When a girl has taken dick with out a condom, the guy came in her. And she liked it, the guy writes pen 15 on her hand. She is now part of the pen 15 club.
John had to take my hand and write pen 15 on my hand at the dentist. Then the receptionist said, "ah huh, pen 15 club." Then i ask what that mean. He tells me then i smack him.