de-, 向下。-cay, 同-cad, 掉落，词源同case, accident. 引申义腐烂。

decay

decay: [15] The notion underlying decay and its close relative decadence is of a ‘falling off’ from a condition of health or perfection. Decay comes from Old Northern French decair, a descendant of Vulgar Latin *dēcadere, which in turn came from Latin dēcidere, a compound verb formed from the prefix dē- ‘down, off, away’ and cadere ‘fall’ (source of English case and a wide range of related words). Decadence [16] was acquired via the medieval derivative dēcadentia. To the same word-family belongs deciduous [17], from Latin dēciduus, literally denoting the ‘falling off’ of leaves from trees.
=> accident, case, chance, decadence, deciduous

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.

decay (n.)

mid-15c., "deterioration, decline in value," from decay (v.). Meaning "gradual decrease in radioactivity" is from 1897.

1. When not removed, plaque causes tooth decay and gum disease.

牙斑如果不清除就会导致牙齿蛀蚀和牙龈疾病。

来自柯林斯例句

2. There are problems of urban decay and gang violence.

存在城市衰败和帮派暴力的问题。

来自柯林斯例句

3. The phosphate was deposited by the decay of marine microorganisms.

海洋微生物腐烂后沉积形成磷酸盐。

来自柯林斯例句

4. The landlord had let the building fall into decay .

房东不管，房子已经破烂不堪。

来自《权威词典》

5. The decay of the meat could have been prevented by proper refrigeration.

如果经过适当的冷藏,这些肉本来不至于腐烂掉.

来自《简明英汉词典》