Of modesty and Margaret Mitchell

One of my favorite scenes in Gone With the Wind ( I LOVE GWTW) is the scene where Scarlett is getting dressed for the Wilkes' barbecue.  Hoping to seduce Ashley Wilkes into marriage, she rejects all her morning dresses, and chooses an afternoon dress that shows off both her arms and decolletage.  Mammy, concerned about propriety, chides Scarlett on her immodest choice (You can't show your bosom before three o'clock!)  The irony, of course, is that the dress, while revealing her cleavage, completely obscures her legs and hips under a bell-shaped crinoline.  Later in the novel, when the crinoline is abandoned in favor of the bustle, Scarlett (who had no problem showing cleavage) felt embarrassed to reveal the shape of her abdomen.
Throughout history, the definition of modesty has been pretty malleable.  Covered heads and legs have been the norm, in some form or another, for at least the past millennium.  However, it has also been considered the norm for women to wear dresses that reveal their busts, especially during evening hours.  During the 19th century, it was considered immodest to wear makeup at all, but in the 1930s, it was considered immodest not to.  It was only during the 1920s that hemlines were raised, and even then, they were raised kicking and screaming.  During the 1960s, pants were unacceptable attire for women in business and social settings, but miniskirts that barely covered a woman's rear were the norm.  In her 1940 edition of Etiquette, Emily Post writes that a business woman's hair must always be held back, not hanging loose like a little girl. 
In the Orthodox world, ankle-length skirts are considered unacceptable for street wear, but stockings are required.  Heads are covered, but with well-made wigs that are indistinguishable from the wearer's natural hair.  The bust must be covered, but few limits are placed on the amount of makeup and jewelry worn.  So, what, pray tell, does it even mean to be "modest"?