(よき)せぬ

よきせぬ
expression, pre-noun adjectival
unexpected; unforeseen; unanticipated
formalwrittendescriptive
1. unexpected; unforeseen; unanticipated
Something that was not anticipated or predicted. A literary/formal expression using the classical negative form せぬ (= しない).
予期(よき)せぬ事態(じたい)()きた
An unforeseen situation occurred.
I was flustered by an unexpected visitor.
You should keep backups in preparation for unforeseen trouble.

Usage

予期(よき)せぬ is a set phrase functioning as a pre-noun modifier. It uses the classical negative form せぬ (equivalent to しない in modern Japanese), giving it a somewhat formal or literary tone.

Common patterns

Always used before a noun: 予期(よき)せぬ + noun. The modern equivalent 予期(よき)しない is also used but sounds less natural as a fixed expression. The form 予期(よき)せぬ is preferred in writing and formal speech.

Forms

Common collocations