They were liberated miscreants rejecting conventional standards of beauty. They were proto-feminists. And the movement didn't begin with rich white girls.
‘The Great Gatsby’ Still Gets Flappers Wrong | Collectors Weekly
In her June 1922 piece for Metropolitan Magazine called “Eulogy on the Flapper,” 22-year-old Zelda only hints at the radical edge of the flapper movement.
“The Flapper awoke from her lethargy of sub-deb-ism, bobbed her hair, put on her choicest pair of earrings and a great deal of audacity and rouge and went into the battle. She flirted because it was fun to flirt and wore a one-piece bathing suit because she had a good figure, she covered her face with powder and paint because she didn’t need it and she refused to be bored chiefly because she wasn’t boring. She was conscious that the things she did were the things she had always wanted to do. Mothers disapproved of their sons taking the Flapper to dances, to teas, to swim and most of all to heart. She had mostly masculine friends, but youth does not need friends—it needs only crowds.”