Source : Webster's Revised Unabridged Dictionary (1913)

Sank \Sank\,
   imp. of {Sink}.

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.

Source : WordNet®

sink
     n 1: plumbing fixture consisting of a water basin fixed to a wall
          or floor and having a drainpipe
     2: (technology) a process that acts to absorb or remove energy
        or a substance from a system; "the ocean is a sink for
        carbon dioxide" [ant: {source}]
     3: a depression in the ground communicating with a subterranean
        passage (especially in limestone) and formed by solution
        or by collapse of a cavern roof [syn: {sinkhole}, {swallow
        hole}]
     4: a covered cistern; waste water and sewage flow into it [syn:
         {cesspool}, {cesspit}, {sump}]
     v 1: fall or drop to a lower place or level; "He sank to his
          knees" [syn: {drop}, {drop down}]
     2: cause to sink; "The Japanese sank American ships in Pearl
        Harbor"
     3: pass into a specified state or condition; "He sank into
        Nirvana" [syn: {pass}, {lapse}]
     4: go under, "The raft sank and its occupants drowned" [syn: {settle},
         {go down}, {go under}] [ant: {float}]
     5: descend into or as if into some soft substance or place; "He
        sank into bed"; "She subsided into the chair" [syn: {subside}]
     6: appear to move downward; "The sun dipped below the horizon";
        "The setting sun sank below the tree line" [syn: {dip}]
     7: fall heavily or suddenly; decline markedly; "The real estate
        market fell off" [syn: {slump}, {fall off}]
     8: fall or sink heavily; "He slumped onto the couch"; "My
        spirits sank" [syn: {slump}, {slide down}]
     9: embed deeply; "She sank her fingers into the soft sand"; "He
        buried his head in her lap" [syn: {bury}]
     [also: {sunken}, {sunk}, {sank}]

sank
     See {sink}