Etymology
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decay (n.)

mid-15c., "deterioration, decline in value, gradual loss of soundness or perfection," from decay (v.). Obsolete or archaic in reference to fortune or property; meaning "decomposition of organic tissue" is from 1590s. In physics, the meaning "gradual decrease in radioactivity" is by 1897.

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decay (v.)

late 15c., "to decrease," also "to decline, deteriorate, lose strength or excellence," 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 "off" (see de-) + Latin cadere "to fall" (from PIE root *kad- "to fall").

Transitive sense of "cause to deteriorate, cause to become unsound or impaired" is from 1530s. Sense of "decompose, rot" is from 1570s. Related: Decayed; decaying.

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*kad- 
Proto-Indo-European root meaning "to fall."

It forms all or part of: accident; cadaver; cadence; caducous; cascade; case (n.1); casual; casualty; casuist; casus belli; chance; cheat; chute (n.1); coincide; decadence; decay; deciduous; escheat; incident; occasion; occident; recidivist.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit sad- "to fall down;" Latin casus "a chance, occasion, opportunity; accident, mishap," literally "a falling," cadere "to fall, sink, settle down, decline, perish;" Armenian chacnum "to fall, become low;" perhaps also Middle Irish casar "hail, lightning."
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putrescence (n.)

"a putrid state; tendency to decay," 1640s, from Latin putrescentem (nominative putrescens), present participle of putrescere "grow rotten, molder, decay," inchoative of putrere "be rotten" (see putrid). Related: Putrescency.

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decadence (n.)

1540s, "deteriorated condition, decay," from French décadence (early 15c.), from Medieval Latin decadentia "decay," from decadentem (nominative decadens) "decaying," present participle of decadere "to decay," from Latin de- "apart, down" (see de-) + cadere "to fall" (from PIE root *kad- "to fall"). Meaning "process of falling away from a better or more vital state" is from 1620s. Used of periods in art since 1852, on French model.

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carious (adj.)
"decayed" (of tooth or bone), 1670s, from French carieux (16c.), from Latin cariosus "full of decay," from caries "rottenness, decay" (see caries). Extended sense of "having a corroded appearance" is by 1832.
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non-perishable (adj.)

also nonperishable, "not subject to rapid decay or deterioration," 1887, from non- + perishable.

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marcescent (adj.)

"withering, liable to decay, ephemeral," 1727, from Latin marcescentem (nominative marcescens), present participle of marcescere "to wither, languish, droop, decay, pine away," inchoative of marcere "to wither, droop, be faint," from Proto-Italic *mark-e-, from PIE root *merk- "to decay" (source also of Sanskrit marka- "destruction, death;" Avestan mareka- "ruin;" Lithuanian mirkti "become weak," merkti "to soak;" Ukrainian dialect morokva "quagmire, swamp," Middle High German meren "dip bread into water or wine," perhaps also Middle Irish mraich, Welsh brag "a sprouting out; malt").

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imperishable (adj.)
"not subject to destruction or decay," 1640s, from assimilated form of in- "not, opposite of" (see in- (1)) + perishable. Related: Imperishably
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rot (n.)

early 14c., "decay, corruption, putrefaction," from rot (v.) or of Scandinavian origin (compare Icelandic rot, Swedish röta, Danish røde "decay, putrefaction"), or both, in any case from the root of the verb. From c. 1400 as the name of a disease in sheep, also generally, "condition of rottenness in a plant or animal, process or state of being rotten." Slang sense of "rubbish, trash" is from 1848.

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