P3N15 is the letters and numbers(in what some call "Leet speak" or "1337 speak") for penis.
-0mg 101z, l1k3 u h4v3 a p3n15!
-Translation: Omg lols, like you have a penis!
-For the old folks: Oh my goodness hahahaha, it has come to my attention that you do not have a penis!
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.
Exclusive club for those in 6th and 7th grade. Only requirement to enter is to have the club's name written on one's body part, such as an arm or hand.
Johnny, join the Pen 15 club! Alls ya need to do is let me write Pen 15 on your arm.