From BiblePortal Wikipedia

Vine's Expository Dictionary of NT Words [1]

1: Παλαιόω (Strong'S #3822 — Verb — palaioo — pal-ah-yo'-o )

"to make old" (palaios), is translated in  Hebrews 8:13 , firstly, "hath made ... old," secondly (Passive Voice), RV "is becoming old" (AV, "decayeth"); "wax old,"  Luke 12:33;  Hebrews 1:11 . See Old.

2: Διαφθείρω (Strong'S #1311 — Verb — diaphtheiro — dee-af-thi'-ro )

"to destroy utterly," as used in  2—Corinthians 4:16 (here in the Passive Voice, lit., "is being destroyed), is rendered "is decaying" (RV, for AV, "perish"). See Corrupt , Destroy.

Webster's Dictionary [2]

(1): ( n.) Destruction; death.

(2): ( v. t.) To destroy.

(3): ( n.) Cause of decay.

(4): ( v. t.) To cause to decay; to impair.

(5): ( v. i.) To pass gradually from a sound, prosperous, or perfect state, to one of imperfection, adversity, or dissolution; to waste away; to decline; to fail; to become weak, corrupt, or disintegrated; to rot; to perish; as, a tree decays; fortunes decay; hopes decay.

(6): ( n.) Gradual failure of health, strength, soundness, prosperity, or of any species of excellence or perfection; tendency toward dissolution or extinction; corruption; rottenness; decline; deterioration; as, the decay of the body; the decay of virtue; the decay of the Roman empire; a castle in decay.

Wilson's Dictionary of Bible Types [3]

 Hebrews 8:13 (b) The Lord uses this strange word to describe the condition of the Old Testament plan and method of dealing with men according to "the law of Moses." The plan failed because of the evil hearts of men. (  Hebrews 7:18-19). Men could not, would not, and did not keep the law. The rebellion in their hearts and the sinfulness of their natures prevented the law from doing for them what should have been done. For that cause the loving GOD of Heaven arranged a new plan entirely and sent Christ Jesus to give the gift of eternal life so that men would be made right inside. Then the outside actions would be according to the Word of GOD.

King James Dictionary [4]

DECA'Y, Fr. dechoir, from L. de and cado, to fall, or decedo.

International Standard Bible Encyclopedia [5]

dē̇ - ´: Although this word is still in good use in both its literal sense, of the putrefaction of either animal or vegetable matter, and its derived sense, denoting any deterioration, decline or gradual failure, the Revised Version (British and American) has replaced it by other expressions in  Leviticus 25:35;  Ecclesiastes 10:18;  Isaiah 44:26;  Hebrews 8:13; in some of these cases with a gain in accuracy of translation. In  Nehemiah 4:10 ( כּשׁל , kāshal , "to be feeble," "stumble") the Revised Version (British and American) retains "is decayed"; in  Job 14:11 ( חרב , ḥārēbh , "to be dried up") the American Standard Revised Version substitutes "wasteth," and in  John 11:39 the American Standard Revised Version has "the body decayeth" instead of the more literal translation offensive to modern ears ( ὄζει , ózei , "emits a smell").