The Collaborative International Dictionary of English v.0.48:

Sink \Sink\ (s[i^][ng]k), v. i. [imp. Sunk (s[u^][ng]k), or
   (Sank (s[a^][ng]k)); p. p. Sunk (obs. Sunken, -- now
   used as adj.); p. pr. & vb. n. Sinking.] [OE. sinken, AS.
   sincan; akin to D. zinken, OS. sincan, G. sinken, Icel.
   s["o]kkva, Dan. synke, Sw. sjunka, Goth. siggan, and probably
   to E. silt. Cf. Silt.]
   1. To fall by, or as by, the force of gravity; to descend
      lower and lower; to decline gradually; to subside; as, a
      stone sinks in water; waves rise and sink; the sun sinks
      in the west.
      [1913 Webster]

            I sink in deep mire.                  --Ps. lxix. 2.
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   2. To enter deeply; to fall or retire beneath or below the
      surface; to penetrate.
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            The stone sunk into his forehead.     --1 San. xvii.
                                                  49.
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   3. Hence, to enter so as to make an abiding impression; to
      enter completely.
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            Let these sayings sink down into your ears. --Luke
                                                  ix. 44.
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   4. To be overwhelmed or depressed; to fall slowly, as so the
      ground, from weakness or from an overburden; to fail in
      strength; to decline; to decay; to decrease.
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            I think our country sinks beneath the yoke. --Shak.
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            He sunk down in his chariot.          --2 Kings ix.
                                                  24.
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            Let not the fire sink or slacken.     --Mortimer.
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   5. To decrease in volume, as a river; to subside; to become
      diminished in volume or in apparent height.
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            The Alps and Pyreneans sink before him. --Addison.
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   Syn: To fall; subside; drop; droop; lower; decline; decay;
        decrease; lessen.
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The Collaborative International Dictionary of English v.0.48:

Sinking \Sink"ing\,
   a. & n. from Sink.
   [1913 Webster]

   Sinking fund. See under Fund.

   Sinking head (Founding), a riser from which the mold is fed
      as the casting shrinks. See Riser, n., 4.

   Sinking pump, a pump which can be lowered in a well or a
      mine shaft as the level of the water sinks.
      [1913 Webster]

WordNet (r) 3.0 (2006):

sinking
    n 1: a descent as through liquid (especially through water);
         "they still talk about the sinking of the Titanic"
    2: a slow fall or decline (as for lack of strength); "after
       several hours of sinking an unexpected rally rescued the
       market"; "he could not control the sinking of his legs"
    3: a feeling caused by uneasiness or apprehension; "with a
       sinking heart"; "a sinking feeling in the pit of my stomach"
       [syn: sinking, sinking feeling]

Moby Thesaurus II by Grady Ward, 1.0:

217 Moby Thesaurus words for "sinking":
   Brownian movement, advance, aging, angular motion, anxious,
   apprehensive, ascending, ascent, axial, axial motion, back,
   back-flowing, backflowing, backing, backward, backward motion, bad,
   baptism, burial, career, climbing, collapsing, coming apart,
   concavity, course, cracking, crumbling, current, de-escalation,
   debasement, decadent, deciduous, declining, declivitous, decurrent,
   deepening, degenerate, degradation, dejected, demotion, depressed,
   depression, descendant, descending, descent, desolate,
   despaired of, despairing, deteriorating, detrusion, digging,
   diminishing, diminution, dip, dipping, disintegrating, doleful,
   dolorous, done for, doting, dousing, down, down-reaching,
   down-trending, downcoming, downfalling, downgoing, downhill,
   downsinking, downward, downward motion, draining, drift, driftage,
   drifting, drilling, drooping, dropping, duck, ducking, dunking,
   dwindling, dying, ebbing, effete, engulfment, excavation, expiring,
   facing death, fading, failing, falling, flagging, flight, flow,
   flowing, fluent, flux, flying, forlorn, forward motion,
   fragmenting, fretful, getting on, given up, going, going to pieces,
   growing old, gyrational, gyratory, hauling down, heavy-laden,
   hollowness, hopeless, immergence, immersion, in articulo mortis,
   in extremis, incapable of life, inundation, jittery, jumpy,
   languishing, low, lowering, marcescent, mining, miserable,
   moribund, mounting, mournful, near death, nervous, nonviable,
   oblique motion, on the descendant, on the downgrade, ongoing,
   onrush, passage, passing, pining, plummeting, plunging, probing,
   progress, progressive, queasy, radial motion, random motion,
   receding, reduction, reflowing, refluence, refluent, reflux,
   regression, regressive, retiring, retreating, retrograde,
   retrogression, retrogressive, rising, rotary, rotational, rotatory,
   run, running, rush, rushing, sagging, senescent, set, setting,
   shaky, shrinking, shriveling, sideward, sideward motion, sinkage,
   sliding, slipping, slipping away, slumping, soaring, souse,
   sousing, sternway, stream, streaming, stricken, submergence,
   submerging, submersion, subsiding, tabetic, tense, terminal,
   thrusting under, tottering, traject, trajet, trend, tumbledown,
   tunneling, uneasy, unquiet, up-trending, upward, upward motion,
   waning, wasting, wilting, withering, woeful, worsening