To deteriorate, to get worse, to lose strength or health, to decline in quality.

“The pair loved to take pictures in the decaying hospital on forty-third street.”

(of organic material) To rot, to go bad.

“The cat's body decayed rapidly.”

(of an unstable atom) To change by undergoing fission, by emitting radiation, or by capturing or losing one or more electrons.

Having undergone decay, rotted.

“The decayed remains of the pilot's shirt showed where the plane had been shot down.”

Reduced in circumstances; having lost one's status or fortune.