Refers to the event of the zipper of the pants bending up giving the impression of a penis in a girl's pants.
Often occurs while sitting down.
Girl: Uck, I can't stand these jeans, they always give me a pants penis when I sit down!
the part up the pants on the zipper that bends up when you sit down to look like a penis. Mostly occurs when wearing jeans.
Girl 1: How did it go with Johnny last night?
Girl 2: It didn't start off too good. When I walked in I thought he was really happy to see me. Turns out it was just a pants penis.