expression
totally unexpected; completely unforeseen
A set phrase used when something happens entirely without warning or beyond prediction. Often carries a nuance of an accident or misfortune.
彼の死は不慮の外だった。
His death was completely unforeseen.
不慮の外の事故で家族を失った。
I lost my family in a totally unexpected accident.
思いがけない is a common adjective meaning 'unexpected' or 'unforeseen', used in everyday speech. 不慮の外 is a more formal, literary set phrase.
A set phrase combining 不慮 (unexpected) with 外 (outside), literally 'outside of expectation'. The exact historical derivation is uncertain, but it functions as a fixed expression for something completely unforeseen.