A club that can only be joined by writing "I love Pen15" on your forearm.
Person 1: Hey! Are you in the club?
Person 2: What club?
Person 1: The pen15 club!
Person 2: Cool! How do I join?
Person 1: Just write "I love Pen15" in your arm!
Person 2: *Writes* Oh shit.
Something that people at the age of 12 or under write on one's body part. It is the ritual in which to get into the PEN15 club, if u write it the correct way, the 1 looks like an I and the 5 looks like an S, so instead of looking like pen with 15 latched onto the end, it spells out PENIS.
Lynn: Wanna join a club?
Caroline: Sure! what club?
Lynn: The PEN15 club! here, this is what it is!
(Lynn writes on Caroline's arm 'PEN15'.)
Caroline: What's PEN15?... OH FUCK!! YOU JUST WROTE PENIS ON MY ARM!!