Yes very interesting anecdote   As far as I can see the word flapper was originally British slang and from another origin but its possible that when the word spread to America folk etymology quickly came up with an explanation for it connected with the fashion for wearing galoshes unbuckled  This association must have happened early on as the  film Bare Knees mentioned above seems already to reflect it Could you add some dates to your anecdote and tell us roughly when your grandmother was wearing this fashion  BeyondMyKen is right this is original research and we shouldnt add it to the article but it could help explain where the galoshes theory for the origin of flapper came from