The Collaborative International Dictionary of English v.0.48:

Decay \De*cay"\, v. t.
   1. To cause to decay; to impair. [R.]
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            Infirmity, that decays the wise.      --Shak.
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   2. To destroy. [Obs.] --Shak.
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The Collaborative International Dictionary of English v.0.48:

Decay \De*cay"\, n.
   1. Gradual failure of health, strength, soundness,
      prosperity, or of any species of excellence or perfection;
      tendency toward dissolution or extinction; corruption;
      rottenness; decline; deterioration; as, the decay of the
      body; the decay of virtue; the decay of the Roman empire;
      a castle in decay.
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            Perhaps my God, though he be far before,
            May turn, and take me by the hand, and more
            May strengthen my decays.             --Herbert.
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            His [Johnson's] failure was not to be ascribed to
            intellectual decay.                   --Macaulay.
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            Which has caused the decay of the consonants to
            follow somewhat different laws.       --James Byrne.
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   2. Destruction; death. [Obs.] --Spenser.
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   3. Cause of decay. [R.]
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            He that plots to be the only figure among ciphers,
            is the decay of the whole age.        --Bacon.

   Syn: Decline; consumption. See Decline.
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The Collaborative International Dictionary of English v.0.48:

Decay \De*cay"\, v. i. [imp. & p. p. Decayed; p. pr. & vb. n.
   Decaying.] [OF. decaeir, dechaer, decheoir, F. d['e]choir,
   to decline, fall, become less; L. de- + cadere to fall. See
   Chance.]
   To pass gradually from a sound, prosperous, or perfect state,
   to one of imperfection, adversity, or dissolution; to waste
   away; to decline; to fail; to become weak, corrupt, or
   disintegrated; to rot; to perish; as, a tree decays; fortunes
   decay; hopes decay.
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         Ill fares the land, to hastening ills a prey,
         Where wealth accumulates and men decay.  --Goldsmith.
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Moby Thesaurus II by Grady Ward, 1.0:

154 Moby Thesaurus words for "decay":
   ablate, ablation, atomization, atomize, atrophy, biodegradability,
   biodegradation, break down, break up, breakup, canker, caries,
   carrion, catalysis, catalyst, collapse, come apart, consume,
   contaminate, corrode, corrosion, corrupt, corruption, crack up,
   crumble, crumble into dust, crumbling, curdle, dandruff,
   debilitate, decadence, decline, decompose, decomposition, decrease,
   defile, degenerate, degeneration, degradability, degradation,
   deteriorate, deterioration, dialysis, dilapidate, dilapidation,
   diminish, disintegrate, disintegration, disjoin, disjunction,
   disorganization, disorganize, dissociation, dissolution, dissolve,
   downfall, dry rot, dwindle, ebb, enfeeble, erode, erosion,
   excrement, fading, failing, fall into decay, fall to pieces,
   ferment, fester, filth, fission, foul matter, foulness, furfur,
   gangrene, go bad, go off, go to pieces, go to pot, go to seed,
   hydrolysis, hydrolyst, incoherence, mess, mildew, mold, molder,
   mortification, mortify, mould, moulder, muck, mucus, necrose,
   necrosis, obscenity, ordure, oxidation, oxidization, perish,
   photolysis, pollute, pus, putrefaction, putrefy, putresce,
   putrescence, putrid matter, putridity, putridness, rancidity,
   rancidness, rankle, rankness, ravages of time, resolution, rot,
   rottenness, rotting, ruin, rust, sap, scurf, scuz, slime, slough,
   smut, snot, sordes, sour, sphacelate, sphacelation, sphacelus,
   split, splitting, spoil, spoilage, suppurate, taint, thermolysis,
   tooth decay, turn, undermine, wane, waste away, wasting, weaken,
   weakening, wear, wear and tear, wear away, wither, work, wreck