It is a rare occurrence when some emasculated male has such a tiny schlong that it gives the illusion that it goes into his body. The closest example to such a body part is an adolescent vagina.
A Real Male: Hey TJ, do you have a vagina or just a NEGATIVE PENIS?
TJ: No! It's just that my dick is so small (and does not satisfy anyone) that it hides in my body like a scared turtle with a small cock. Therefore people consider it NEGATIVE.
when someones penis is so small that it comes out their asshole. Commonly used as an insult.
Bob: Hey markm, your penis is so small, you've got a negative penis going on!
Mark: Shut the hell up Bob