The Collaborative International Dictionary of English v.0.48:

Flop \Flop\ (fl[o^]p), v. t. [imp. & p. p. Flopped (fl[o^]pt);
   p. pr. & vb. n. Flopping.] [A variant of flap.]
   1. To clap or strike, as a bird its wings, a fish its tail,
      etc.; to flap.
      [1913 Webster]

   2. To turn suddenly, as something broad and flat. [Colloq.]
      --Fielding.
      [1913 Webster]