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.
   [1913 Webster]

         Ill fares the land, to hastening ills a prey,
         Where wealth accumulates and men decay.  --Goldsmith.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Decayed \De*cayed"\, a.
   Fallen, as to physical or social condition; affected with
   decay; rotten; as, decayed vegetation or vegetables; a
   decayed fortune or gentleman. -- De*cay"ed*ness, n.
   [1913 Webster]

WordNet (r) 3.0 (2006):

decayed
    adj 1: damaged by decay; hence unsound and useless; "rotten
           floor boards"; "rotted beams"; "a decayed foundation"
           [syn: decayed, rotten, rotted]