Before the 1920's women dressed in very long skirts with shirts that had collars, they also had to keep their hair very long. As WWI neared they began to change their appearance in small ways.
This is how they dressed prior to the 1920's and the first World World War.
Many people saw WWI as being started by the mistakes and beliefs of the older generation, they saw so much death as a soldier or as being a nurse that tended to the wounded. young women wanted to move away from the old ways that they believed brought death. After the war was over it was very difficult to adjust to life the way it was before. They decided to change their appearance, by cutting their hair, being more revealing, and rebelling against their elders.
The term "flapper" was first used in England to describe young women who had not yet fully matured. Authors such as F. Scott Fitzgerald described them as "lovely, expensive, and nineteen." They were also noted to trim down all of their clothing to make moving around a lot easier. Their outer clothing which was called "garconne" or "little boy." To look more like a boy they would wind strips of cloth around their chest to flatten it. All of this characterized the new women of the 20's who rejected the old ways.
Information on the Flappers: history1990s.about.com/od/1990s/a/flappers.htm