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decay (v.)[decay 词源字典]: 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 etymology, decay origin, 英语词源]
deciduous (adj.): 1680s, from Latin deciduus "that which falls off," from decidere "to fall off," from de- "down" (see de-) + cadere "to fall" (see case (n.1)). Originally with reference to leaves, petals, teeth, etc.; specific sense of "trees whose leaves fall off" (opposed to evergreen) is from 1778.
degenerate (v.): 1540s, from Latin degeneratus, past participle of degenerare "fall from ancestral quality" (see degenerate (adj.)). Figurative sense of "to fall off, decline" was in Latin. Related: Degenerated; degenerating.