The kanji 朽 generally conveys the meaning of decay, rot, or decomposition, specifically referring to the process of something becoming rotten or crumbling due to age, time, or the elements. It suggests a state of deterioration and the gradual breakdown of material.

・On-yomi (音読み): キュウ (kyū) ・Kun-yomi (訓読み): く朽ちる (kuchiru) – to rot, to decay

⚫︎ 老朽化（ろうきゅうか） Translation: aging; deterioration; becoming run-down ⚫︎ 木朽 (き朽) Translation: decayed wood; rotten wood ⚫︎ 腐朽（ふきゅう） Translation: decay; decomposition; rotting

The kanji 朽 is a 形声文字 (keisei moji), a phono-semantic compound character. It combines the radical 木 (ki, meaning "tree" or "wood") with the phonetic component 不 (fu, meaning "not" or "without"). The combination suggests that the tree is "not" healthy or whole anymore, but rather in a state of decay. The phonetic component不 contributes to the sound of the kanji.