flapper

Possibly from Victorian sporting slang, meaning a young fledgling bird which attempts to fly by flapping its wings (see this sense under etymology 2 below), perhaps in particular wildfowl in August which are full-sized, tender and worthwhile quarry, but are naive and unable to fly properly due to the late development of flight feathers in ducks and geese.

Alternative derivations are also suggested. The word flap was slang in the 17th century for a prostitute (an extension of the sexual slang meaning “vulva”), and by the late 19th century in England flapper could mean either a very young prostitute or a teenage girl too old to be a child and too young to be considered 'out' in society.

By the early 20th century, the word was commonly understood as a description of hairstyle: “A 'flapper', we may explain, is a young lady who has not yet been promoted to long frocks and the wearing of her hair 'up'.”