Source : Webster's Revised Unabridged Dictionary (1913)

Sink \Sink\, v. i. [imp. {Sunk}, or ({Sank}); 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.

            I sink in deep mire.                  --Ps. lxix. 2.

   2. To enter deeply; to fall or retire beneath or below the
      surface; to penetrate.

            The stone sunk into his forehead.     --1 San. xvii.
                                                  49.

   3. Hence, to enter so as to make an abiding impression; to
      enter completely.

            Let these sayings sink down into your ears. --Luke
                                                  ix. 44.

   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.

            I think our country sinks beneath the yoke. --Shak.

            He sunk down in his chariot.          --2 Kings ix.
                                                  24.

            Let not the fire sink or slacken.     --Mortimer.

   5. To decrease in volume, as a river; to subside; to become
      diminished in volume or in apparent height.

            The Alps and Pyreneans sink before him. --Addison.

   Syn: To fall; subside; drop; droop; lower; decline; decay;
        decrease; lessen.

Sinking \Sink"ing\,
   a. & n. from {Sink}.

   {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.

Source : WordNet®

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 feeling}]