A flapper was a term for women that lived in Earth's Roaring Twenties and were known for a particular style of clothing.
In 2370, flappers were part of the Orient Express holograms seen by members of USS Enterprise-D's crew. (TNG: "Emergence")
In 2372, the artwork for the front cover of The New included a flapper dancing with a gentleman. (VOY: "Death Wish")
In 2375, a Silver Blood copy of Tom Paris described the kind of "down-to-Earth" 1920s Chicago honeymoon he had in mind for himself and B'Elanna Torres as consisting of "Speakeasies, flappers, the Charleston." (VOY: "Course: Oblivion")