Any injury that results in a loose flap of skin on the fingers, making gripping difficult.
Possibly from Victorian sporting slang, meaning young 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: 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: "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'".
The earliest documented use in the sense of "attractive young girl" is in the 1903 novel Sandford of Merton by Desmond Coke: "There's a stunning flapper.". The word also suggested a spirited girl of unconventional or mischievous disposition. An advertisement in the The Times reads: "The father of a young lady, aged 15 – a typical “FLAPPER” – with all the self assurance of a woman of 30 would be grateful for the recommendation of a seminary (not a convent) where she might be placed for a year or two with the object of taming her."
By 1912 the word had apparently both crossed the Atlantic and evolved to mean a slightly older girl: British stage impresario John Tiller defined it for readers of the New York Times as meaning "a girl who has just "come out". She is at an awkward age, neither a child nor a woman...". The word had clearly caught on, as a Mme. Nordica is quoted using it in the New York Times of January 1, 1913: "...a thin little flapper of a girl donning a skirt in which she can hardly take a step, extinguishing all but her little white teeth with a dumpy bucket of a hat..."
By 1920 in England it clearly meant any young woman of a pleasure-seeking disposition: a Dr R. Murray-Leslie criticized "the social butterfly type...the frivolous, scantily-clad, jazzing flapper, irresponsible and undisciplined, to whom a dance, a new hat, or a man with a car, were of more importance than the fate of nations."