When a girl knows she sweats more during sex than her man and does not care. When the act is done she continues to remain on top of her man and smother him with her sweat until she naturally dries off.
A "dirty Kelly" is when you go out in public completely naked late at night with a girl who doesn't mind having sex on the hood of your car, a park bench, or the neighbor's trampoline.
I went for a dirty Kelly behind the Walmart last night with a chick I met on the clearance aisle.