The lower half, or skirt, ranges in length from above the knees to the floor.
"Skirt" comes from "skyrta", an old Norse word for "shirt", so it seems appropriate to call an entire dress a skirt and not just the lower half.
Skirt is also slang for a woman, often one who is morally loose.
"Mel: 'What the hell is that'?
Cher: 'A dress'.
Mel: 'Says who'?
Cher: 'Calvin Klein'."
Looks like underwear to me!
Why do women walk around half-nude and men don't do that? You don't see men wearing shorts to the office like women often wear mini-skirts to work.