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.
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            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.
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   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.
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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]