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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.
by NigelMarsflap June 04, 2010
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