Green’s Dictionary of Slang

1. a male hairstyle, in which the hair is worn low over the brow.

1909

J. Ware Passing Eng. of the Victorian Era 3/2: The younger costers wear rival forehead tufts – such as the Quiff, the Guiver, or the Flop. There is, however, one golden rule for these fashions – the hair must stop short of the eyelids.

2. a similar female hairstyle.

1909

J. Ware Passing Eng. of the Victorian Era 134/2: Flop (Low Lond., 1881). When the lower classes of women adopted the ‘cretin’ or ‘poodle’ style of wearing the hair low down over the forehead, they gave it this name.