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]

The Collaborative International Dictionary of English v.0.48:

Flop \Flop\ (fl[o^]p), v. i.
   1. To strike about with something broad and flat, as a fish
      with its tail, or a bird with its wings; to rise and fall;
      as, the brim of a hat flops.
      [1913 Webster]

   2. To fall, sink, or throw one's self, heavily, clumsily, and
      unexpectedly on the ground. [Colloq.] --Dickens.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Flop \Flop\, n.
   Act of flopping. [Colloq.] --W. H. Russell.
   [1913 Webster]

WordNet (r) 3.0 (2006):

flop
    adv 1: with a flopping sound; "he tumbled flop into the mud"
    2: exactly; "he fell flop on his face" [syn: right, flop]
    n 1: an arithmetic operation performed on floating-point
         numbers; "this computer can perform a million flops per
         second" [syn: floating-point operation, flop]
    2: someone who is unsuccessful [syn: flop, dud, washout]
    3: a complete failure; "the play was a dismal flop" [syn:
       flop, bust, fizzle]
    4: the act of throwing yourself down; "he landed on the bed with
       a great flop" [syn: flop, collapse]
    v 1: fall loosely; "He flopped into a chair"
    2: fall suddenly and abruptly
    3: fail utterly; collapse; "The project foundered" [syn: fall
       through, fall flat, founder, flop]

The Free On-line Dictionary of Computing (19 January 2023):

FLOP

   1. An early system on the IBM 701.

   [Listed in CACM 2(5):16 (May 1959)].

   (1994-11-14)

   2. Erroneous singular of FLOPS.

   (2005-06-17)


The Devil's Dictionary (1881-1906):

FLOP, v.  Suddenly to change one's opinions and go over to another
party.  The most notable flop on record was that of Saul of Tarsus,
who has been severely criticised as a turn-coat by some of our
partisan journals.