When look at the penis externally, the flap of mars is where the scrotum curls around the side of the shaft, leaving a flap which doesn't move without prodding or poking.
Man 1: I've got a big dick.
Man 2: Yeah, but The Flap of Mars in your pants is below average in terms of flaccidity.
Man 1: ...damn.
Man 2: Yeah, but The Flap of Mars in your pants is below average in terms of flaccidity.
Man 1: ...damn.
by NigelMarsflap June 4, 2010