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:

Sink \Sink\ (s[i^][ng]k), v. t.
   1. To cause to sink; to put under water; to immerse or
      submerge in a fluid; as, to sink a ship.
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            [The Athenians] fell upon the wings and sank a
            single ship.                          --Jowett
                                                  (Thucyd.).
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   2. Figuratively: To cause to decline; to depress; to degrade;
      hence, to ruin irretrievably; to destroy, as by drowping;
      as, to sink one's reputation.
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            I raise of sink, imprison or set free. --Prior.
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            If I have a conscience, let it sink me. --Shak.
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            Thy cruel and unnatural lust of power
            Has sunk thy father more than all his years. --Rowe.
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   3. To make (a depression) by digging, delving, or cutting,
      etc.; as, to sink a pit or a well; to sink a die.
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   4. To bring low; to reduce in quantity; to waste.
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            You sunk the river repeated draughts. --Addison.
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   5. To conseal and appropriate. [Slang]
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            If sent with ready money to buy anything, and you
            happen to be out of pocket, sink the money, and take
            up the goods on account.              --Swift.
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   6. To keep out of sight; to suppress; to ignore.
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            A courtly willingness to sink obnoxious truths.
                                                  --Robertson.
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   7. To reduce or extinguish by payment; as, to sink the
      national debt.
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The Collaborative International Dictionary of English v.0.48:

Sink \Sink\ (s[i^][ng]k), n.
   1. A drain to carry off filthy water; a jakes.
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   2. A shallow box or vessel of wood, stone, iron, or other
      material, connected with a drain, and used for receiving
      filthy water, etc., as in a kitchen.
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   3. A hole or low place in land or rock, where waters sink and
      are lost; -- called also sink hole. [U. S.]
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   4. The lowest part of a natural hollow or closed basin whence
      the water of one or more streams escapes by evaporation;
      as, the sink of the Humboldt River. [Western U. S.]
      [Webster 1913 Suppl.]

   Sink hole.
      (a) The opening to a sink drain.
      (b) A cesspool.
      (c) Same as Sink, n., 3.
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WordNet (r) 3.0 (2006):

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, sink,
       swallow hole]
    4: a covered cistern; waste water and sewage flow into it [syn:
       cesspool, cesspit, sink, sump]
    v 1: fall or descend to a lower place or level; "He sank to his
         knees" [syn: sink, 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: sink, pass, lapse]
    4: go under, "The raft sank and its occupants drowned" [syn:
       sink, settle, go down, go under] [ant: float,
       swim]
    5: descend into or as if into some soft substance or place; "He
       sank into bed"; "She subsided into the chair" [syn: sink,
       subside]
    6: appear to move downward; "The sun dipped below the horizon";
       "The setting sun sank below the tree line" [syn: dip,
       sink]
    7: fall heavily or suddenly; decline markedly; "The real estate
       market fell off" [syn: slump, fall off, sink]
    8: fall or sink heavily; "He slumped onto the couch"; "My
       spirits sank" [syn: slump, slide down, sink]
    9: embed deeply; "She sank her fingers into the soft sand"; "He
       buried his head in her lap" [syn: bury, sink]

Moby Thesaurus II by Grady Ward, 1.0:

499 Moby Thesaurus words for "sink":
   Babylon, Gomorrah, KO, Sodom, abate, ablate, advance, age,
   alveolation, alveolus, antrum, aquamanile, armpit, ascend,
   automatic dishwasher, back, back up, ball up, bankrupt, baptize,
   basin, bate, bath, bathtub, be clobbered, be consumed,
   be eaten away, be felled, be gone, be lost, be poleaxed,
   be staggered, be stricken, bear down, beat down, become suicidal,
   bend, bidet, blow, bog, bollix, bollix up, bore, bowl, break,
   break down, bring low, brothel, budge, bugger, bugger up, burn out,
   burrow, bury, bust, buy in, buy into, capsize, cast down, cave,
   cave in, cavity, cease, cease to be, cease to exist, cesspit,
   cesspool, change, change place, cheat the undertaker, circle,
   climb, cloaca, cloaca maxima, collapse, come apart, come down,
   come to grief, come unstuck, concave, concavity, conk out, consume,
   consume away, cook, corrode, couch, countersink, crack up, crater,
   crumble, crypt, cup, damp, dampen, dampen the spirits, darken,
   dash, debase, decline, decrease, deepen, defeat, degenerate,
   deject, deliquesce, deluge, delve, dematerialize, demit, den,
   den of iniquity, depart, depress, depression, descend, despair,
   despond, deteriorate, detrude, die, die away, die out, dig,
   dig out, dike, diminish, dip, disappear, discourage, dish,
   dishearten, dishpan, dishwasher, disimprove, disintegrate, dispel,
   disperse, dispirit, dissipate, dissolve, dive, do a fade-out,
   do for, do in, dodder, douse, downbear, drain, dredge, drift away,
   drill, drive, droop, drop, drop off, drown, duck, dump, dunk,
   dwindle, ebb, engulf, erode, evanesce, evaporate, ewer, excavate,
   exit, expire, fade, fade away, fade out, fail, faint, fall,
   fall away, fall in, fall off, fatigue, financier, finger bowl, fix,
   fizzle out, flag, flee, fleet, fleshpots, flit, flop, flop down,
   flow, flump, flump down, fly, fold, follicle, font, foul up,
   founder, funnel chest, furrow, garbage dump, gasp, get, get along,
   get on, get over, get through to, get tired, give out, give way,
   go, go around, go away, go down, go downhill, go off, go round,
   go sideways, go soft, go to pieces, go to pot, go under, gouge,
   gouge out, groove, grow old, grow weary, grub, gum up, gutter,
   gyrate, hash up, haul down, have a comedown, have a mishap, head,
   headchute, hellhole, hide, hit a slump, hit rock bottom,
   hit the skids, hole, hollow, hollow shell, immerge, immerse,
   impoverish, incline, indent, inundate, invest, invest in, jade,
   joint, keel, keel over, kennel, kitchen sink, knock down,
   knock out, lacuna, languish, lapse, lavabo, lavatory,
   lay out money, lead, lean, leave no trace, leave the scene, lessen,
   let, let down, let up, lose heart, lose strength, louse up, lower,
   lower oneself, lower the spirits, make an investment, marsh, melt,
   melt away, melt like snow, merge, mess up, mine, mire, mount, move,
   move away, move off, move over, muck up, oppress, overset,
   overturn, overwhelm, pandemonium, pant, pass, pass away, pass out,
   peak, peg out, penetrate, perish, peter out, pine, piscina, pit,
   pitchpole, place, play hell with, play hob with, play out, plop,
   plop down, plow back into, plumb the depths, plummet, plump,
   plunge, plunge in water, pocket, point, poop out, precipitate,
   press down, progress, puff, puff and blow, pull away, pull down,
   punch bowl, push down, put, put down, quagmire, quarry, queer,
   reach the depths, recede, reduce, register, regress, reinvest,
   retire, retire from sight, retreat, retrocede, retrograde,
   retrogress, rise, risk, rot, rotate, ruin, run, run aground,
   run down, run low, run out, sadden, sag, sap, scoop, scoop out,
   scrabble, scrape, scratch, screw up, scupper, scuttle, septic tank,
   set, settle, settle down, sewer, shake, shell, shift, shipwreck,
   shoot down, shovel, shower, shower bath, shower curtain,
   shower head, shower room, shower stall, showers, shrink, shrivel,
   sink away, sink down, sink in, sink like lead, sink money in,
   sink of corruption, sinkage, sinkhole, sinus, slide, slip,
   slip away, slouch, sluice, slump, slump down, snafu, snarl up,
   soar, socket, somersault, sough, souse, spade, speculate, spin,
   stand off, stir, stoop, stoup, stream, sty, submerge, submerse,
   subside, succumb, suffer a misfortune, suffer an eclipse, sump,
   swag, swamp, tail off, take down, tend, tend to go, thrust down,
   tip over, tire, torpedo, totter, touch bottom, travel, trench,
   trough, tub, tunnel, turn gray, turn over, turn turtle, turn white,
   undo, upset, upset the boat, vanish, vanish from sight, venture,
   vug, wane, wash barrel, wash boiler, washbasin, washbowl, washdish,
   washer, washing machine, washing pot, washpot, washstand, washtub,
   waste, waste away, weaken, wear, wear away, wear thin, weary,
   weigh heavy upon, weigh upon, wheeze, whelm, whirl,
   widen the distance, wilt, withdraw, wither, wither away, wizen,
   worsen, wreck, wrinkle, yield