decay, rot
朽ちる（くちる）、不朽（ふきゅう）、老朽（ろうきゅう）
to decay, immortal/eternal, old/decrepit
On-reading (キュウ) is used in compounds; kun-reading (く(ちる)) is used standalone or with okurigana. Appears in many compounds — memorize the key ones rather than the kanji in isolation. This kanji forms verbs/adjectives with okurigana. Practice both the word form and compound forms.
