Flop \Flop\, v. t. [imp. & p. p. Flopped; 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.

2. To turn suddenly, as something broad and flat. [Colloq.]

--Fielding.

Source: Webster's Revised Unabridged Dictionary (1913)