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“decay” in Latin

The English word decay itself goes back to Latin cadō, through *dēcadere.

Riddle & Arnold A Copious and Critical English–Latin Lexicon, 1864

decay v.
INTRANS., in ruinam pronum esse (PROP., to be near falling): labi (PROP., to fall in; e.g., of a house, of the cheeks, then figuratively to sink, to become worse; e.g., of a breed or race, of manners, of sciences): collabi (to be falling to ruins; PROP., of buildings; figuratively, of the state): dilabi (PROP., to separate from each other; e.g., of the parts of buildings, of walls; then figuratively to be going downwards; e.g., of the state): macrescere (to fall away, of living beings): tabescere (to waste away, of living beings): marcescere (fade away; hence, languish, grow weak; e.g., marcescentes senio vires, Plin.; marcescere desidia, Liv.): deminui (of the moon, also of strength; it would be wrong to use diminui, Vid: Ochs., Cic., Ecl., p. 13): minui; minuere (to be lessened; e.g., of the memory): my strength decays or fails me, viribus senesco, or deficior; vires extenuantur, deficiunt: my memory decays, memoria labat, minuitur; deficior memoria: zeal and hope decay together, studium cum spe senescit: the power of the Athenians decays, opes Atheniensium senescunt: to decay or be reduced to poverty, ad inopiam or egestatem or paupertatem redigi.
TRANS., imbecillum or infirmum reddere; attenuare; vitiare (see WEAKEN). A decayed tooth, densexesus: dens cavus or cavatus (hollow): decayed (= reduced to poverty), ad inopiam, egestatem or paupertatem redactus: beams that are decayed, asseres marcidi (Vitrvius).
decay
deminutio: defectio virium; vires corporis affectae: of the understanding, deminutio mentis. To fall to decay, to go to decay. Vid. DECAY, v. INTR.

Quick answer

What is the Latin word for decay?
The usual Latin words for “decay” are lābor, collābor, dīlābor; also macrēscō, tābēscō, marcēscō. The right choice depends on the sense; see the distinctions below.

Entry from J. E. Riddle & T. K. Arnold, A Copious and Critical English–Latin Lexicon (rev. C. Anthon, New York: Harper, 1864), digitised by Fergus J. P. Walsh, CC BY-SA 4.0; Latin markup adapted by latindictionary.co.