Synonyms for Decayed:

adj.

all (adjective)
rotted, rotten.
deteriorated (adjective)
degenerate, dilapidated, Declined, atrophied, retrogressed, putrefied, corrupt, downgraded, Deteriorated.
disintegrated (adjective)
dissected, dilapidated, Degenerated, gangrenous, festering, decomposed, mortified, dissolved, rotten, putrefied, molten, crumbled, broken, shattered, Anatomized, spoiled, Disintegrated, turned.
rotten, falling apart (adjective)
putrefied, wasted, putrid, gangrenous, withered, rotted, decomposed, spoiled.
shriveled (adjective)
withered.
wasted (adjective)
wasted, despoiled, residual.

n.

putrid. decayed (noun)
unsound, rotted, rotten.

v.

deteriorated (verb)
sunk, tergiversated, Slipped, Slid, collapsed, gotten worse, went downhill, Sank, Regressed, wilted, Ebbed, fell apart, faded, Waned, slumped, withered, Reverted, got worse, Relapsed, mouldered, gone downhill, lapsed, worsened, fallen apart.
disintegrated (verb)
fractured, rotted, ruptured, broke.

Other synonyms:

corrupt
putrid.
Other relevant words:
putrid, unsound.

Usage examples for decayed

  1. Horses very often have decayed teeth, and suffer with toothache. – Pratt's Practical Pointers on the Care of Livestock and Poultry by Pratt Food Co.
  2. It was brought to Kirkwall, but could not be preserved, in consequence of its decayed condition. – Notice of Runic Inscriptions Discovered during Recent Excavations in the Orkneys by James Farrer
  3. Aye, replied Hiram, dragging off a great bunch of it from what looked like the decayed trunk of one of the oak trees, hollowed out by age and exposure to the heavy tropical rains of the region, thet's what they calls the orchilla weed, I guess. – The Island Treasure by John Conroy Hutcheson