1. A vulgar term which means the male penis and the female vagina; basically saying that the penis and vagina are best friends.
2. Your 'person' who will be there for you no matter what. Who shares their secrets, and laughs at your corny inside jokes and practically lives at your house. Who, if hurt by you, wil tell you to your face, and apologize if they hurt you. If they don't either you're wrong, or they're just not your best friend
guy: hey ya know, we're best friends in a way
girl: (thinks to herself: AH HELL NAH!)
in which way?
guy: because of our genitals! my penis is your vagina's best friend! they're the best friends, right?
girl: (thinks: OMIFRIGGINGOSH!)
*SLAP, SLAP* YOU ARE SUCH A PERV AND AN ASSHOLE!