noun-modifying (irregular)
Prenominal form of 由々しい, used only before nouns. Describes a situation, matter, or problem that is very serious and cannot be ignored. Often used in formal or written contexts.
See also: 由々しい
これは由々しき問題だ。
This is a grave problem.
由々しき事態が発生した。
A serious situation has occurred.
深刻 is a more common and neutral word for 'serious' or 'grave', while 由々しき is more formal and emphatic, often implying a sense of alarm or urgency.
The prenominal form of 由々しい (yuyushii), an adjective meaning 'grave' or 'serious'. The kanji 由 means 'reason' or 'cause', but the exact derivation of the word is uncertain. The repetition of the kanji (由々) intensifies the meaning.