This stands for P-E-N-I-S..As you can see the 15 at the end of pen15...the 1 resembles the "i" and the 5 resembles the "s". Lots of grade schoolers ask about, "do u have a pen15?" they say that its a pen, so when they say "no" they start to humiliate the person. This is a very imature act.
Yo sup? hey do u have a pen15?
WHAT?!? whats a pen15
oh its a kewl pen, all the boys have them
well, do u have 1?