A fork of Rural Dictionary
"Pen15" is h4x0r writing for "Penis." It is used most commonly to write it on each other's hands (kids do this) in order to bring about lame ass ridiculing and taunting. It's a very childish thing to do and isn't really funny...
Kid1: Wanna join the Pen15 Club?
Kid2: What are the requirements?
Kid1: I just have to write it on your hand and then you're in *giggle*