When you and a friend have sex with the same
girl in the course of three years or 100
bangs, whichever comes first, you share a bond created by your penis in the
girl. You are therefore penisiblings.
Bro
1:
Dude, last night I got laid with that chick Tiffany
Bro 2: No way! We're penisiblings!
Bro
1: High five!
*The sound of this high five brings happiness to everyone in a 2 miles radius, and makes them say "Nice"*