Synonyms for Decaying:

adj.

rotting (adjective)
breaking down, spoiling, falling to pieces, wasting away, Collapsing, breaking up, Disintegrating, wearing away, Oxidizing, Crumbling, rusting, eroding, deteriorating, decomposing.
scrubby (adjective)
down-at-heel, all the worse for wear.

n.

shoddy, dingy, better, tattered, ratty, down-at-heel, rundown, dy, scruffy, threadbare, faded, bedraggled, mangy, sleazy, scrubby, shabby, all the worse for wear, tatty, broken-down, dilapidated, tacky, decrepit. rotting (noun)
falling to pieces, rusting, wasting away, Oxidizing, dying, wearing away, breaking up, rotten, tumbling down, eroding, breaking down, falling.

v.

deteriorating (verb)
going downhill, mouldering, deteriorating, Dilapidating, atrophying, ebbing, withering, Slumping, slipping, getting worse, sliding, downgrading, sinking, falling apart, waning, retrogressing, regressing, lapsing, declining, relapsing, reverting, tergiversating, fading, Collapsing, worsening, wilting.
disintegrating (verb)
Fracturing, shattering, decomposing, Anatomizing, Dissecting, Disintegrating, Degenerating, turning, Rupturing, rotting, breaking, mortifying, Crumbling, Putrefying, dissolving, spoiling.

Other synonyms:

down-at-heel, scruffy, ratty, tatty, rotten. sleazy, shabby, scrubby, mangy, tattered, faded, bedraggled, dingy, dying. decrepit, shoddy. rundown. threadbare. broken-down. run-down
dilapidated.
Other relevant words:
threadbare, eroding, broken-down, rotten, all the worse for wear, tatty, faded, tacky, shabby, tattered, rundown, dilapidated, decrepit, scruffy, scrubby, dying, down-at-heel, mangy, dingy, rusting, sleazy, bedraggled, shoddy, ratty.

Usage examples for Decaying

  1. Surely, then, your real life has lain in the justice you have done, the wrongs you have righted, the trust you have built up in the people's hearts, and not in these decaying walls which no enemy ever threatened in your time nor for a hundred years before you came." – Major Vigoureux by A. T. Quiller-Couch
  2. There are boxes of fruit under the bed, some of it decaying what is too rotten to sell must serve for home consumption. – White Slaves by Louis A Banks
  3. On the other hand, the extent to which the strong odour, which was observed at the place and resembled that of burned horns, arose from the decaying mammoth remains, is perhaps uncertain. – The Voyage of the Vega round Asia and Europe, Volume I and Volume II by A.E. Nordenskieold