decay
v.
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.
n.
mid-15c., "deterioration, decline in value," from decay, v.. Meaning "gradual decrease in radioactivity" is from 1897.
〔李〕[de-=down下；cay（cad的变形）=to fall降→] v. fall down from a state of health and excellence;rot衰退；腐朽