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]

Moby Thesaurus II by Grady Ward, 1.0:

317 Moby Thesaurus words for "flop":
   Grand Guignol, Passion play, Tom show, also-ran, alter, ameliorate,
   antimasque, audience success, bag, ballet, bang, bankrupt,
   be a gas, be a hit, be changed, be converted into, be renewed,
   beat, beating, bed, bicker, bill, bomb, bottom out, break,
   broadcast drama, bump, burlesque show, burst, bust, cascade, cave,
   cave in, change, charade, checker, chop, chop and change, clanger,
   clap, clash, cliff hanger, closet drama, collapse, come about,
   come around, come round, come to naught, come to nothing,
   comedy drama, crack, crap out, crash, crawl in, critical success,
   cropper, crump, culbute, daggle, dance, dangle, daytime serial,
   debacle, decline, degenerate, depend, deteriorate, deviate,
   dialogue, disaster, dive, diverge, diversify, documentary drama,
   doss down, drabble, drag, draggle, drama, dramalogue,
   dramatic play, dramatic series, dramatize, drape, draw a blank,
   droop, drop a bomb, drop down, drop the ball, dud, dull thud,
   duodrama, duologue, epic theater, experimental theater,
   extravaganza, fail, failure, fall, fall down, fall flat,
   false alarm, fan, feature, fiasco, fizzle, flap, flat failure,
   flick, flicker, flip, flit, flitter, flop down, floperoo, flow,
   flummox, flump, flump down, flutter, fold, fold up, forced landing,
   founder, frost, gasser, get left, give way, giveaway, go down,
   go pitapat, gutter, hang, hang down, happening, haul around,
   header, headline, hit, hit show, hit the hay, hit the sack,
   improve, improvisational drama, jibe, kip down, knock, lapse,
   lay an egg, legitimate drama, lemon, lop, lose out, loser, lower,
   make a hit, masque, meliorate, melodrama, melodramatize,
   minstrel show, miracle, miracle play, mitigate, modulate,
   monodrama, monologue, morality, morality play, mount, music drama,
   musical revue, mutate, mystery, mystery play, nod, not hack it,
   not make it, open, open a show, opera, pageant, palpitate,
   palpitation, panel show, pantomime, pastoral, pastoral drama, pend,
   piece, pile in, pitapat, pitter-patter, play, playlet, plop,
   plop down, plump, plunge, pratfall, premiere, present, preview,
   problem play, produce, psychodrama, pulse, put on, quiver,
   quiz show, radio drama, rap, report, review, revive, revue,
   roll in, sack out, sack up, sag, scenarize, sensational play,
   serial, set, set the stage, settle, settle down, shake, shift,
   show, sink, sink down, sitcom, situation comedy, sketch, skit,
   slam, slap, slat, slouch, slump, slump down, smack, soap,
   soap opera, sociodrama, spectacle, spill, splat, splutter, sprawl,
   sputter, stage, stage play, stage show, star, straight drama,
   strike out, stumble, submerge, subside, succeed, success,
   suspense drama, swag, swap, swerve, swing, tableau, tableau vivant,
   tack, take a turn, take the count, talk show, tap, teleplay,
   television drama, television play, theater of cruelty,
   theatricalize, throb, thwack, topple, total loss, total theater,
   trail, trip, try out, tumble, turkey, turn, turn aside, turn in,
   turn into, turn the corner, undergo a change, variety show, vary,
   vaudeville, vaudeville show, veer, vehicle, warp, wash out,
   washout, wave, waver, weep, whack, wham, whap, whiff, whomp, whop,
   word-of-mouth success, work, worsen



The Free On-line Dictionary of Computing (30 December 2018):

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.