The act of not wearing underwear, for females. Just as mud flaps behind the wheels of a truck flail wildly in the wind, a saggy vagina may mimic this flapping motion without the restriction of underwear. This is the alternative to "free balling" meant for female use only.
"Dude, it's my laundry day, so I'm mud-flappin' it today."
by PACHFI May 05, 2010
The act of a female "going commando." It can get pretty messy down there, so it is up to the labium majora and labium minora to flap around freely and fight off any kind of dirt or grime.
Jon: "Hey Sally, wanna do the dirty?"

Sally: (removes pants) "What kind of dirty?"

Jon: "Eww God. It looks like you rubbed oatmeal all over your vagina!"

Sally: "I wouldn't doubt it... I have been mudflappin' all week"

Jon: "Now I see why they call it the dirty, baby."
by Tough Tucca November 14, 2011

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