late 15c., "to decrease," from Anglo-French decair, Old North French decair (Old French decheoir, 12c., Modern French déchoir) "to fall, set (of the sun), weaken, decline, decay," from Vulgar Latin *decadere "to fall off," from de- (see de-) + Latin cadere "to fall" (see case (n.1)). Meaning "decline, deteriorate" is c.1500; that of "to decompose, rot" is from 1570s. Related: Decayed; decaying.
mid-15c., "deterioration, decline in value," from decay (v.). Meaning "gradual decrease in radioactivity" is from 1897.
decay de·cay (dĭ-kā')
The destruction or decomposition of organic matter as a result of bacterial or fungal action; rot.
The loss of information that was registered by the senses and processed into the short-term memory system.
To break down into component parts; rot.
To disintegrate or diminish by radioactive decay.
To decline in health or vigor; waste away.
Verb To undergo decay.