a gradeschool prank usually you would write:

Pen
15

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!
by maxzilla April 25, 2005
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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.
by Misty Dawson September 02, 2006
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