The Collaborative International Dictionary of English v.0.48:

Slump \Slump\, n. [Cf. D. slomp a mass, heap, Dan. slump a
   quantity, and E. slump, v.t.]
   The gross amount; the mass; the lump. [Scot.]
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Slump \Slump\, v. t. [Cf. Lump; also Sw. slumpa to bargain for
   the lump.]
   To lump; to throw into a mess.
   [1913 Webster]

         These different groups . . . are exclusively slumped
         together under that sense.               --Sir W.
                                                  Hamilton.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Slump \Slump\, v. i. [imp. & p. p. Slumped; p. pr. & vb. n.
   Slumping.] [Scot. slump a dull noise produced by something
   falling into a hole, a marsh, a swamp.]
   1. To fall or sink suddenly through or in, when walking on a
      surface, as on thawing snow or ice, partly frozen ground,
      a bog, etc., not strong enough to bear the person.
      [1913 Webster]

            The latter walk on a bottomless quag, into which
            unawares they may slump.              --Barrow.
      [1913 Webster]

   2. To slide or slip on a declivity, so that the motion is
      perceptible; -- said of masses of earth or rock.
      [Webster 1913 Suppl.]

   3. To undergo a slump, or sudden decline or falling off; as,
      the stock slumped ten points. [Colloq.]
      [Webster 1913 Suppl.]

The Collaborative International Dictionary of English v.0.48:

Slump \Slump\, n.
   1. A boggy place. [Prov. Eng. & Scot.]
      [1913 Webster]

   2. The noise made by anything falling into a hole, or into a
      soft, miry place. [Scot.]
      [1913 Webster]

   3. A falling or declining, esp. suddenly and markedly; a
      falling off; as, a slump in trade, in stock market prices,
      in a batter's average, etc. [Colloq.]
      [Webster 1913 Suppl. +PJC]

WordNet (r) 3.0 (2006):

slump
    n 1: a noticeable deterioration in performance or quality; "the
         team went into a slump"; "a gradual slack in output"; "a
         drop-off in attendance"; "a falloff in quality" [syn:
         slump, slack, drop-off, falloff, falling off]
    2: a long-term economic state characterized by unemployment and
       low prices and low levels of trade and investment [syn:
       depression, slump, economic crisis]
    v 1: assume a drooping posture or carriage [syn: slump,
         slouch]
    2: fall or sink heavily; "He slumped onto the couch"; "My
       spirits sank" [syn: slump, slide down, sink]
    3: fall heavily or suddenly; decline markedly; "The real estate
       market fell off" [syn: slump, fall off, sink]
    4: go down in value; "the stock market corrected"; "prices
       slumped" [syn: decline, slump, correct]

Moby Thesaurus II by Grady Ward, 1.0:

219 Moby Thesaurus words for "slump":
   arrear, arrearage, arrears, bad times, be found wanting,
   bear market, bearish market, beat down, boom, bottoming out, break,
   business cycle, business fluctuations, bust, cadence, catabasis,
   catenary, cave, cave in, cheapen, cheapening, collapse, come short,
   comedown, cooling off, crash, crisis, cut, cut prices, debasement,
   decadence, decadency, deceleration, declension, declination,
   decline, decline and fall, declining market, decrescendo,
   decurrence, defalcation, default, defectiveness, deficit, deflate,
   deflation, deformation, degeneracy, degenerateness, degeneration,
   degradation, delinquency, demotion, depravation, depravedness,
   depreciate, depreciation, depression, derogation, descend, descent,
   deterioration, devaluate, devaluation, devolution, die, diminuendo,
   dip, dive, downslide, downswing, downtrend, downturn,
   downward mobility, downward trend, droop, drop, dwindling, dying,
   ebb, economic cycle, economic expansion, economic growth,
   economic stagnation, effeteness, evil day, expanding economy,
   expansion, fade, fading, fail, failing, failure, failure of nerve,
   fall, fall away, fall in price, fall off, fall short,
   falling short, falling-off, falloff, flag, flop, flop down, flump,
   flump down, founder, give way, go down, go downhill, go off,
   growth, hard times, heavy weather, high growth rate, hit a slump,
   hit rock bottom, hit the skids, imperfection, inadequacy,
   inferiority, insufficiency, involution, jew down, keel over, lack,
   lag, lapse, lose ground, loss of tone, low, lower, lowering,
   mark down, markdown, market expansion, nose dive, nose-dive,
   nosedive, not answer, not hack it, not make it, not make out,
   not measure up, not stretch, not suffice, off market, pare, peak,
   peaking, pitch, plop, plop down, plummet, plummeting, plump,
   plunge, price cut, price fall, price reduction, prosperity,
   rainy day, reach the depths, recede, recession, recovery, reduce,
   reduction, regression, remission, retreat, retreating market,
   retrocession, retrogradation, retrogression, run down, run short,
   sad times, sag, sagging market, set, settle, settle down, shave,
   short measure, shortage, shortcoming, shortfall, sink, sink down,
   sinkage, slash, slide, slip, slippage, slouch, slowdown,
   slump down, smash, soft market, stagnation, stop short,
   stormy weather, submerge, submergence, subside, subsidence, swag,
   tailspin, topple, touch bottom, trim, trough, tumble, underage,
   upturn, wane, want