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:

Sunken \Sunk"en\, a.
   Lying on the bottom of a river or other water; sunk.
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WordNet (r) 3.0 (2006):

sunken
    adj 1: having a sunken area; "hunger gave their faces a sunken
           look" [syn: deep-set, sunken, recessed]

Moby Thesaurus II by Grady Ward, 1.0:

60 Moby Thesaurus words for "sunken":
   boat-shaped, boatlike, bowl-shaped, bowllike, buried, cavelike,
   cavernous, concave, concaved, craterlike, cup-shaped, cupped,
   cymbiform, debased, depressed, dish-shaped, dished, dishing,
   dishlike, downcast, downthrown, drawn, drowned, engulfed, fallen,
   flooded, funnel-breasted, funnel-chested, funnel-shaped, haggard,
   hollow, hollowed, immersed, incurved, incurving, incurvous,
   infundibular, infundibuliform, inundated, low, lowered, navicular,
   naviform, prostrate, reduced, retiring, retreating, saucer-shaped,
   scaphoid, scyphate, settled, spoonlike, subaqueous, submarine,
   submerged, submersed, sunk, underground, undersea, underwater