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!!
Lynn: Haha, Sara showed me!
Caroline: you mean Sara from 6th grade? you talked to someone in the grade ahead of us?
Lynn: yeah, she wrote it on my arm first...