The Collaborative International Dictionary of English v.0.48:

Flounder \Floun"der\, n. [Cf. Sw. flundra; akin to Dan. flynder,
   Icel. fly?ra, G. flunder, and perh. to E. flounder, v.i.]
   1. (Zool.) A flatfish of the family Pleuronectid[ae], of
      many species.
      [1913 Webster]

   Note: The common English flounder is Pleuronectes flesus.
         There are several common American species used as food;
         as the smooth flounder (P. glabra); the rough or
         winter flounder (P. Americanus); the summer flounder,
         or plaice (Paralichthys dentatus), Atlantic coast;
         and the starry flounder (Pleuronectes stellatus).
         [1913 Webster]

   2. (Bootmaking) A tool used in crimping boot fronts.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Flounder \Floun"der\, v. i. [imp. & p. p. Floundered; p. pr. &
   vb. n. Floundering.] [Cf. D. flodderen to flap, splash
   through mire, E. flounce, v.i., and flounder the fish.]
   To fling the limbs and body, as in making efforts to move; to
   struggle, as a horse in the mire, or as a fish on land; to
   roll, toss, and tumble; to flounce.
   [1913 Webster]

         They have floundered on from blunder to blunder. --Sir
                                                  W. Hamilton.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Flounder \Floun"der\, n.
   The act of floundering.
   [1913 Webster]

WordNet (r) 3.0 (2006):

flounder
    n 1: flesh of any of various American and European flatfish
    2: any of various European and non-European marine flatfish
    v 1: walk with great difficulty; "He staggered along in the
         heavy snow" [syn: stagger, flounder]
    2: behave awkwardly; have difficulties; "She is floundering in
       college"

Moby Thesaurus II by Grady Ward, 1.0:

120 Moby Thesaurus words for "flounder":
   agonize over, alternate, back and fill, be all thumbs, be at sea,
   be hard put, be uncertain, beat about, blunder, blunder away,
   blunder into, blunder on, blunder upon, boggle, botch, bumble,
   bungle, butcher, capsize, careen, career, come a cropper,
   commit a gaffe, doubt, ebb and flow, fall, fall down, fall flat,
   fall headlong, fall over, fall prostrate, falter, faux pas,
   feel unsure, flounce, fluctuate, fumble, get a cropper,
   go through phases, grope, grovel, have trouble, heave, hobbyhorse,
   labor, labor under difficulties, list, lumber, lurch,
   make heavy weather, mar, miscue, muddle, muff, murder, oscillate,
   pendulate, pitch, pitch and plunge, pitch and toss,
   play havoc with, plunge, pound, puzzle over, question, rear, reel,
   ring the changes, rock, roll, scend, seesaw, seethe, shift,
   shuffle, slip, spoil, sprawl, spread-eagle, stagger, strive,
   struggle, stumble, sway, swing, take a fall, take a flop,
   take a header, take a pratfall, take a spill, teeter,
   teeter-totter, thrash about, tilt, toil, topple, topple down,
   topple over, toss, toss and tumble, toss and turn, totter, travail,
   trip, tumble, turn, turn turtle, vacillate, vary, volutation,
   walk on eggshells, wallop, wallow, waver, wax and wane, welter,
   wobble, wonder, wonder whether, yaw